Don't allow image_list in create project endpoint

This fixes a problem that was introduced in this commit [1]. The call to
`skip_load_resource only: :create` was removed which meant that the call
to `load_and_authorize_resource` triggered an attempt to instantiate a
`Project` model using the params identified by CanCanCan. This included
the `image_list` param which did not exist as an attribute on a
`Project` and thus resulted in the following exception:

    ActiveModel::UnknownAttributeError: unknown attribute 'image_list' for Project.

I did initially look at fixing the problem while maintaining the ability
to pass the `image_list` into `Project::Create#call`. However, the
functionality in `Project::Create#build_project` which loops through the
`image_list` was not previously tested and I can't really see that it
ever worked. Furthermore, @sra405 has pointed out that we're not
currently allowing users to add their own images to a project for
safeguarding reasons and so I thought it made more sense to fix the
problem by removing support for `image_list` from the create project
endpoint for now until we actually need it. /cc @loiswells97

Removing the `image_list` param from the permitted params in
`Api::ProjectsController#base_params` should mean it is never included
in the params supplied to either CanCanCan's
`load_and_authorize_resource` method or to `Project::Create#call`,
although it will mean that a warning will appear in the logs. So ideally
we'd remove it from the params being supplied by the UI.

Note that the images from the projects in the `raspberrypilearning`
GitHub organisation [2] are added to a project using a different
mechanism and so should not be affected by this change.

[1]: 39f45f7

Author: floehopper

app/controllers/api/projects_controller.rb

@@ -84,7 +84,6 @@ def base_params
         :project_type,
         :locale,
         {
-          image_list: [],
           components: %i[id name extension content index default]
         }
       )

lib/concepts/project/operations/create.rb

@@ -18,13 +18,8 @@ def call(project_hash:)
 
       def build_project(project_hash)
         identifier = PhraseIdentifier.generate
-        new_project = Project.new(project_hash.except(:components, :image_list).merge(identifier:))
+        new_project = Project.new(project_hash.except(:components).merge(identifier:))
         new_project.components.build(project_hash[:components])
-
-        (project_hash[:image_list] || []).each do |image|
-          new_project.images.attach(image.blob)
-        end
-
         new_project
       end
     end

spec/concepts/project/create_spec.rb

@@ -27,7 +27,6 @@
             content: 'print("hello world")',
             default: true
           }],
-          image_list: [],
           user_id:
         }
       end