Show project index status in the Admin UI

Background

Customers are having difficultly understanding what to do when code doesn’t show the expected results. Or it's unclear on why they may not be getting results.

Proposal

There are two rake tasks that customers can use to identify missing projects in the index. This could be ported to the Admin UI

• gitlab:elastic:index_projects_status
• gitlab:elastic:projects_not_indexed

backend changes

1. Move the code called by the rake task into new Services that can be called from rake tasks and frontend
2. add calls to the services to return
   • List of projects not indexed
   • Count of items in the incremental queue
   • Count of items in the initial queue

frontend changes

1. Create a new section for indexing status
2. Do not show the section (or make the calls) in the following cases:
   • if on GitLab.com
   • if indexing is disabled - We probably don't want to load this page if indexing isn't enabled.
3. Display list of un-indexed projects with created date, count of items in incremental queue, and count of items in initial queue
4. Display help which explains that newly created projects may not be indexed immediately with a link to documentation