Add missing ResourceLinkEvent#synthetic_note_class

What does this MR do and why?

Add missing ResourceLinkEvent#synthetic_note_class. In the case of the ResourceLinkEvent we do not need a new note, so to avoid the NoMethodError, we decided to make the note optional by the returning false on #synthetic_note_class.

This also adds tests to ensure new classes that inherits from ResourceEvent implements this method to avoid problems.

Related to:

• #467243 (closed)
• !114394 (merged)
• !107584 (merged)

Changelog: fixed

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Before	After

How to set up and validate locally

1. Enable okrs_mvc feature flag - https://docs.gitlab.com/ee/user/okrs.html
2. Go to a project and create a OKR issue (project > issues > on the "new issue" button click on the arrow and select new OKR)
3. Add children to the new OKR (On the children table, click in add new Objective or Key result, and create a new child)
4. Remove one of the child OKR/KR:
   • without the fix, this will raise a NoMethodError exception
   • with this fix, no exception is raised