* Prepare factories and associations for Users::DeleteActivity specs
* Altered user associations, delete the needed associations in Users::DeleteActivity
* Moved and refined user deletion spec
* Removed useless commented user associations
* Refactored BackupData factory
* Refactored ProfilePin factory
* Refactored Reaction factory
* Fixed specs: Keep default _type in factories or specify it in tests
* Ok, notes shouldn't be deleted on user deletion
* Fixed creating reactable for specs
* Specify commentable/commentable_type explicitly when creating data for tests
* Fixed typo in the comments spec
* Removed unnneeded space
* Added aggregate_failures for testing user associations deletion
* Keep feedback_messages while deleting user activities
* added a condition to check if owner of the comment is in mentions
* added a test to check if comment owner can mention self
* fixed metion test by:
- adding a second user to simulate a owner of a comment
- change the user on comment to owned by newly created user
* Implemented ActiveJob for Mention#send_email_notification
* Implemented ActiveJob for Mention.create_all
* Fixed args while calling the job
* Changes as per the review
