Add Article post_commit when user_id changes with specs

[lsparlin]

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

Description

When an article user_id (Author) is changed by an Admin, NotificationSubscriptions belonging to that article should start sending to the new author instead of the old one.
This pull request adds an after_update_commit to be called only when the user_id changes that will update all the NotificationSubscriptions belonging to the article.

Related Tickets & Documents

Closes #14886

QA Instructions, Screenshots, Recordings

• Create a post/article (or find one)
• Log in as an Admin
• Change the author ID on the post through the Admin interface
• Observe that new comments on the article create notifications for the new Use rand not the original

Added/updated tests?

• Yes
• No, and this is why: please replace this line with details on why tests
  have not been included
• I need help with writing tests

[Forem core team only] How will this change be communicated?

Will this PR introduce a change that impacts Forem members or creators, the
development process, or any of our internal teams? If so, please note how you
will share this change with the people who need to know about it.

• I've updated the Developer Docs or
  Storybook (for Crayons components)
• This PR changes the Forem platform and our documentation needs to be
  updated. I have filled out the
  Changes Requested
  issue template so Community Success can help update the Admin Docs
  appropriately.
• I've updated the README or added inline documentation
• I've added an entry to
  CHANGELOG.md
• I will share this change in a Changelog
  or in a forem.dev post
• I will share this change internally with the appropriate teams
• I'm not sure how best to communicate this change and need help
• This change does not need to be communicated, and this is why not: please
  replace this line with details on why this change doesn't need to be
  shared

[github-actions]

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[msarit]

With two accounts open (one in an incognito tab), I tested as follows:

• Published an article as User1, commented on said article as User2, and ensured that User1 got the notification
• Changed the author of article from user1 to User2, commented on said article as User1, and ensured that (1) User1 received no notifications (2) User2 got the notifications

Changes work as expected. 🚀