Add explicit licensing for Elasticsearch

What does this MR do?

Introduces a new EES license feature - GitLab_ElasticSearch - and gates all elasticsearch functionality on that feature being available.

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

Is the empty state (hide the elasticsearch settings entirely) OK for now?

Why was this MR needed?

See https://gitlab.com/gitlab-org/gitlab-ee/issues/2335

Screenshots (if relevant)

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?

Closes #2581 (closed)