apps/notifications - create notification system

[shn-liqd]

Tasks

• PR name contains story or task reference
• Documentation (docs and inline)
• Tests (including n+1 and django_assert_num_queries where applicable)
• Changelog

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[m4ra]

Overall the abstraction approach is good. However, here are some important points:

• Make a simpler file structure, because using same folder names as the app names (projects, comments, etc) is prone to import errors and confusing. I highly recommend keeping all these new notification classes in just two files -> the strategies/base.py and strategies/mixin.py - see my inline comment on the project.py about mixin vs inheritance.
• Keep all signals in one file too, and the helpers.py move it under the notifications folder. for same reason of keeping the overall file structure easier to navigate.
• Do the inheritance from object, and not from ABC, as object is a more standard way to work with models, than just abstract methods in django.
• Consider removing the Strategy suffix off the class names, as it only make the names longer without adding semantic value.
• Verify the correct parameters are passed via the helper function _create_notifications for the strategies
• Include in the docs, the custom template tag usage, and why is it useful.