add a cc quick action for issuable

@smcgivern

What does this MR do?

Add's a CC quick action that creates a system note and todo for each CC'ed user.

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

I had to comment out the inclusion of BlocksJsonSerialization in the user model, otherwise I always got a serialization disabled message when trying to post a /cc @user comment. Maybe there's a necessary change that I missed?

Also, I could not get the tests to run properly locally via the docs that I was able to find about them.

We'd discussed adding a cc service, but when I sat down to work on this it felt like having CC in the base issuable and common system notes classes made more sense. Let me know if this is okay.

Why was this MR needed?

to add the cc quick action

Screenshots (if relevant)

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?

24615