Do not raise error in AutocompleteController#users when not authorized

What does this MR do?

Fix 500 error in https://gitlab.com/gitlab-org/gitlab-ce/issues/25031

• Changelog entry added
• 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 #25031 (closed)

Marked the task Changelog entry added as completed

Added 1 commit:

• 99463f82 - Do not raise error in AutocompleteController#users when not authorized

Compare with previous version

Added ~164274 label

Added 1 commit:

• 73cf8555 - Do not raise error in AutocompleteController#users when not authorized

Compare with previous version

Resolved all discussions

Looks good to me, thanks! Passing to @rymai

Reassigned to @rymai

Added 1 commit:

• f30aeedc - Do not raise error in AutocompleteController#users when not authorized

Compare with previous version

Resolved all discussions

Thanks @artofhuman, LGTM!

Enabled an automatic merge when the build for f30aeedc succeeds

Added 44 commits:

• f30aeedc...4a3a4131 - 43 commits from branch gitlab-org:master
• f5e8337c - Do not raise error in AutocompleteController#users when not authorized

Compare with previous version

Marked the task All builds are passing as completed

Milestone changed to %8.15

Status changed to merged

Mentioned in commit 2706550a