Fix cross referencing for private and internal projects

What does this MR do?

It fixes cross referencing objects that are not publicly visible.

It sets current_user correctly whenever extractor is used in all_references (Mentionable concern)

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

Why was this MR needed?

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7926/diffs#056b58f8b11b5ed98472bf7ee291814409cbde33_49_51 we started using memorised extractor. However when called for the first time from NewNoteWorker the current_user is nil - that is correct.

However when it is called for the second time - when trying to create cross references - it should be set to note.author. But we use already initialised extractor with nil as current_user.

There is a permission check as the next step. But for current_user = nil all objects that are not public are evaluated as not accessible to the user.

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 #31978 (closed)