Make `gfm_auto_complete` into a module and fix up tech debt

What does this MR do?

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1881

• Make gfm_auto_complete into a module (instead of global gl.GfmAutoComplete)
• Tidy up some tech debt

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

• See app/views/layouts/application.html.haml for the init_auto_complete move in order to get data available when we boot up.

Does this MR meet the acceptance criteria?

• Tests
  • 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