Add global wiki search using Elasticsearch

What does this MR do?

Uses Elasticsearch to provide global wiki search

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

We should pay special attention to information exclusivity and specs for that. In particular, project features have been problematic in the past.

Why was this MR needed?

Requested feature

Screenshots (if relevant)

https://gitlab.com/gitlab-org/gitlab-ee/uploads/7dc2f66c2c5b6e5468d394996323e788/Screenshot_from_2017-04-26_13-07-09.png

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 #1565 (closed)