Repository synchronization between primary and secondary nodes

Implement repository synchronization for Gitlab Geo: #76 

- [x] pushes on primary node should notify secondaries
- [x] primary node should buffer notification and send them in batches
- [x] primary node should pool regularly notification queue and notify secondaries in batches
- [x] API endpoint to receive batched notification to update projects
- [x] secondary nodes should not accept push from anyone
- [x] geo nodes should have own ssh keys to be able to sync repositories
- [x] secondary nodes should fetch repositories from primary
- [x] admin screen to manage geo nodes

See merge request !179

Fix Projects::ImportService spec



See merge request !247

Improved specs

Fixed non-fast-forward fetch

Will git fetch with "--prune"

(temporarily disabling any operations to anex as it's currently not
replicated)