Add elastic rake tasks to check group indexing status

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Problem to solve

After adding Group Wiki and Epic to the index, the gitlab:elastic rake tasks need a way for administrators to check on group indexing status

Proposal

• Add a new rake task for index_groups_status to match index_projects_status
• Add a new rake task for groups_not_indexed to match projects_not_indexed
• Add documentation for both new tasks
• Consider adding a third task to show overall index status and run all related tasks for group and projects

Some ideas for implementation considerations:

• These tasks are likely not usable on GitLab.com due to SQL performance
• Do the work in the RakeTaskExecutorService to be called by the rake tasks so it can easily be added to the UI in a future MR