Fix notifications' exceptions

[voodoorai2000]

Relate PR

• Notifications for hidden resources Notifications for hidden resources #2172

Why

• Some users are experiencing exceptions when viewing their notifications

What

• Refactors notification system
• Tests every combination of in-app notifications with the different notifiable models
• Gotchas solved:
  • Notifications for resources that have been hidden by a moderator
  • Notifications for resources that have been retired by the author
  • Notifications for resources that did not have a created_at column

How

• Using a concern to extract the logic common to every notifiable resource
• Using shared_examples to test model specs and feature specs related to notifications for each resource that is notifiable through in-app notifications (🔔 )

Test

• Staging environment testing will be required to make sure all users can now see their notifications

Deployment

• As usual

Warnings

• Proposal Notifications are a special case. Tests are duplicated. It should be refactored in a separate PR. Probably best once we continue working on Admin Notifications and seeing how these two types would fit in nicely with the exiting Notifications about comments and comment replies.