Prior to this commit, the `index_statuses` table was allowed to go stale after
initial indexing. This made it impossible to provide status details, and also
made treating the initial index as a 'backfill' job harder.

Now, if a repository is indexed via `git push`, the initial indexing will
recognise that it has been indexed, and not re-index it.

We introduce an asynchronous initial indexer and an index status rake task at
the same time.