ES: Global code search

• Basic implementation of blobs search
• Basic implementation of commits search
• Fix pagination for commits. Do paginating on ES level instead of resulting array
• Refactoring. The gitlab-elastcsearch-git gem should not know anything specific to GitLab application structure.
• Backport changes to CE
  • Search result parsers (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6341)
  • Removing total_count (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6216)
• Make rake tasks pass parent id
• Test database search afterwards
• Mention somewhere that ES should be updated to 2.4.0
• Specs
• Address TODO in the code
• Add project name to search results

Related MR for gitlab-elasticsearch-git gem (gitlab-elasticsearch-git!18 (merged))

Screenshots:

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/556