Support advanced search queries using elasticsearch

What does this MR do?

Switch to using the simple_query_string query tool in elasticsearch, enabling advanced search.

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

What about the frontend? It should be obvious to the user when ES is enabled (and so the advanced search terms will work), and they should be able to get quick help on the syntax.

Is this search syntax acceptable? It seems to match the google-style search syntax specified in the issue.

Any other problems with using this querier? It looks like a drop-in replacement that just happens to do exactly what we want!

Why was this MR needed?

Respecting exact queries is a deliverable for 9.2: https://gitlab.com/gitlab-org/gitlab-ee/issues/2037 and we are interested in other advanced search syntax too, per https://gitlab.com/gitlab-org/gitlab-ee/issues/1052

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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 #2037 (closed) #1052 #1732 (closed)