Fix issue detail if user who last edited an issue was deleted

What does this MR do?

It adds a fallback to Editable concern for a case when an issue was edited but last_edited_by is nil. This happens when a user who edited the issue was deleted. Additionally it migrates last_edited_by and merge_user (merge user does not cause a problem but should be migrated anyway) to the "Ghost user" when deleted. This will affect only future deletion of users therefore the fallback is still necessary.

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

Why was this MR needed?

When a user who last edited an issue has been deleted an error page is displayed because we called issuable.last_edited_by.name and last_edited_by was nil in that case.

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #34930 (closed)