Add per page to user autocomplete (EE)

What does this MR do?

Uses the per_page parameter to determine how many users to return EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11742

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

Shouldn't be

Why was this MR needed?

Fix API inconsistency

Screenshots (if relevant)

None

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?

https://gitlab.com/gitlab-org/gitlab-ce/issues/32977