EE: Refactor AutocompleteController

What does this MR do?

This is the EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20908.

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)