Import the gitlab-elasticsearch-git gem

What does this MR do?

Copies the code from the gitlab-elasticsearch-git gem into the main gitlab-ee repository

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

I didn't copy the specs as the cases all seemed to be covered within the main repository, and integrating the support code would have been difficult.

I considered moving the code into app/models/concerns/elastic, but this would have overcomplicated matters for bin/elastic_repo_indexer

Why was this MR needed?

Keeping this code in a separate repository is unnecessary and overly complicates MRs such as https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1770#note_28673005

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

What are the relevant issue numbers?

Closes gitlab-elasticsearch-git#6 (closed)