Delete notifications when deleting comments

[vbrandl]

This PR implements a new function find_for_comment on Notification to list all notifications for a specific comment. The delete function on Comment is changed to list and delete all connected notifications.

I wasn't able to test the changes yet since I don't have the test environment set up, so the PR is WIP for now.

This should close #463 and close #500.

[codecov]

Codecov Report

Merging #499 into master will decrease coverage by 0.05%.
The diff coverage is 19.04%.

@@            Coverage Diff             @@
##           master     #499      +/-   ##
==========================================
- Coverage   26.89%   26.83%   -0.06%     
==========================================
  Files          65       65              
  Lines        8966     9000      +34     
==========================================
+ Hits         2411     2415       +4     
- Misses       6555     6585      +30

[vbrandl]

The following changes have been made:

• Mentions inside comments are now properly inserted into the database (comment_id is set, instead of post_id)
• Notifications for mentions are deleted, when the comment gets deleted
• Notifications for comments enabled

The following use cases were tested:

• Comment on a post, check for notification and delete comment again -> notification gets deleted too
• Mention one or multiple user(s) in a comment, check notifications for all users and delete comment again -> notifications for all users are deleted

[trinity-1686a]

Mentions inside comments are now properly inserted into the database (comment_id is set, instead of post_id)

Does this mean it fixes #500 too?

[vbrandl]

No. That's another issue. In that issue, mentioning and mentioned user are confused somewhere, I guess.

[vbrandl]

It actually looks like this also closes #500.

[elegaanz]

It is a bit weird to receive two notifications for the same comment when you are mentioned in a comment under your own article, but it is not really related to this PR, and otherwise it works great. Thank you. 😊