Add projects:create_experience_cs_example rake task & run as part of Heroku release (#532)

This new rake task creates a single example project for the Experience
CS app. If the project with the same `identifier` & `locale` already
exists, the task is a no-op, i.e. the task is idempotent and can be
safely run multiple times.

We look for the project based on just the `identifier` & `locale`,
because there is validation and a database constraint that requires the
combination of these two attributes to be unique. Thus if we don't find
a project, we should be able to create one without an exception being
raised.

We've used a separate rake task rather than incorporating it into the
existing `projects:create_all` rake task for now, because there's some
uncertainty about how that decides whether to overwrite a project. By
keeping it completely separate we avoid iterating over all the projects
and potentially unintentionally changing other projects that already
exist in the editor-api database.

We haven't included this new rake task in `db/seeds.rb`, because it's
only needed in development by the Experience CS team; not by the Code
Editor for Education team.

The projects represented by the data in the sub-directories under
[`lib/tasks/project_components`][1] in this repo are the ones imported
by the existing `projects:create_all` rake task via
[`ProjectImporter#import!`][2]. We've confirmed that the selected
identifier, 'experience-cs-example' does *not* exist in those
sub-directories, which means we can be confident the new rake task won't
accidentally overwrite one of these projects in the editor-api database.

The projects represented by the repos under [the `raspberrypilearning`
GitHub organisation][3] are the ones whose import is triggered by GitHub
webhook requests to [`GitHubWebhooksController#github_push`][4] via
[`UploadJob#perform`][5] and thence the same `ProjectImporter#import!`
method as before. We've confirmed that the selected identifier,
'experience-cs-example' [does *not* exist][5] in any repos in the
`raspberrypilearning` GitHub organisation, which means we can be
confident that the new rake task won't accidentally overwrite one of
these projects in the editor-api database.

We have to use `Project.unscoped` in the new rake task in order to
determine whether the example project already exists, because of the
default scope on `Project` that we added in
#513.

We explicitly set the `project_type` to `Project::Types::SCRATCH`, i.e.
'scratch', so that it will only be found when a request is made to [the
`Api::ProjectsController#show` action][7] with [the 'project_type` param
set to 'scratch'][8]. This means that the Code Editor for Education
should never "see" Scratch projects like this example one.

We explicitly set the `locale` to 'en' which seems to be the default
[set in `FilesystemProject.import_all!`][9] and one of [the defaults
specified in `ProjectLoader`][10]. This seems to match up with the
projects defined in `lib/tasks/project_components` none of which set a
value for the 'LOCALE' key in their `project_config.yml` file.

We explicitly set the `user_id` to `nil`, even though this is the
default, because we want to make it clear that this example project to
should be visible to users that are *not* signed in.

In #527, the
`projects:create_all` rake task invocation was removed from the Heroku
release, because of concerns it might reset/overwrite existing projects
in the database.

Now that we have a separate & idempotent
`projects:create_experience_cs_example` rake task with much more limited
scope, we're confident that it's safe to invoke this new rake task as
part of the Heroku release.

Closely related to
RaspberryPiFoundation/experience-cs#55.

[1]:
https://github.com/RaspberryPiFoundation/editor-api/tree/491c72859f66b49d249d6b0186f002e70673cd2a/lib/tasks/project_components
[2]:
https://github.com/RaspberryPiFoundation/editor-api/blob/491c72859f66b49d249d6b0186f002e70673cd2a/lib/project_importer.rb#L18-L28
[3]: https://github.com/raspberrypilearning
[4]:
https://github.com/RaspberryPiFoundation/editor-api/blob/491c72859f66b49d249d6b0186f002e70673cd2a/app/controllers/github_webhooks_controller.rb#L6-L8
[5]:
https://github.com/RaspberryPiFoundation/editor-api/blob/491c72859f66b49d249d6b0186f002e70673cd2a/app/jobs/upload_job.rb#L57-L75
[6]:
https://github.com/search?q=org%3Araspberrypilearning+experience-cs-example&type=code
[7]:
https://github.com/RaspberryPiFoundation/editor-api/blob/491c72859f66b49d249d6b0186f002e70673cd2a/app/controllers/api/projects_controller.rb#L19-L27
[8]:
https://github.com/RaspberryPiFoundation/editor-api/blob/491c72859f66b49d249d6b0186f002e70673cd2a/app/controllers/api/projects_controller.rb#L72
[9]:
https://github.com/RaspberryPiFoundation/editor-api/blob/491c72859f66b49d249d6b0186f002e70673cd2a/app/models/filesystem_project.rb#L19
[10]:
https://github.com/RaspberryPiFoundation/editor-api/blob/491c72859f66b49d249d6b0186f002e70673cd2a/lib/project_loader.rb#L8

Author: floehopper

Procfile

@@ -1,3 +1,3 @@
 web: bundle exec puma -C config/puma.rb
-release: bundle exec rails db:migrate
+release: bundle exec rails db:migrate && bundle exec rake projects:create_experience_cs_example
 worker: bundle exec good_job start --max-threads=8

lib/tasks/projects.rake

@@ -5,4 +5,22 @@ namespace :projects do
   task create_all: :environment do
     FilesystemProject.import_all!
   end
+
+  desc "Create example Scratch project for Experience CS (if it doesn't already exist)"
+  task create_experience_cs_example: :environment do
+    attributes = {
+      identifier: 'experience-cs-example',
+      locale: 'en',
+      project_type: Project::Types::SCRATCH,
+      name: 'Experience CS Example',
+      user_id: nil
+    }
+    if Project.unscoped.exists?(attributes.slice(:identifier, :locale))
+      puts 'Scratch project already exists'
+    elsif Project.create(attributes)
+      puts 'Scratch project created successfully'
+    else
+      puts 'Scratch project creation failed'
+    end
+  end
 end