WIP: Resolve "Highlighting of notifications in activity stream"

What does this MR do?

Update the four dashboards that show activity events in various levels: (project scope, group scope, a specific user, current user's global scope), so that events that the current user has been notified about are highlighted.

If a user has received any email notification about an Issue, Merge Request, Snippet or comment (Note) on any of the aforementioned entities, then any Event presented in the four Activity Dashboards that references the Entity is highlighted.

Are there points in the code the reviewer needs to double check?

Related to the newly introduced method SentNotification.retrieve_noteables_that_match_with_events

• Is this the correct place to add such a support method according to GitLab's guidelines?
• Should Rspec tests be added for this method?

Additional reason this is labeled as WIP:

• Needs approval by UX team.
• I used the bs-callout-success class in order to highlight the events. I am pretty sure that there are better style classes to be used.

Screenshots

Project activity with highlighted events:

Group scope activity with highlighted events:

(Global) Activity (Dashboard):

User profile page with recent activity:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #13745 (moved)