Add User autocompletion to global search bar

What does this MR do and why?

Adds users to the autocomplete search bar so that it returns users matching the search term.

This is the first release of the feature:

1. User autocompletion for CE using database queries 👈 this MR
2. User autocompletion for EE using elasticsearch if available

Screenshots or screen recordings

user_autocomplete_demo

How to set up and validate locally

1. Go to your local server e.g. http://127.0.0.1:3000
2. In the search bar, start typing the name of a user in your database and see if it pops up in the autocomplete bar under the Users section
3. Go to a project's page and do the same
4. Go to a group's page and do the same

Related to #377530 (closed)