Lock issuables

What does this MR do?

It supports locking issues and merge requests. If an issue/an MR is locked only team members can create new notes or edit the old ones. The users who are not team members can't add new comments or edit the old ones.

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

Why was this MR needed?

requested more than a year ago

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

What are the relevant issue numbers?

Closes #18608 (closed)