Instant Search: Provide dashboard of indexing and sync progress

Instant Search relies on a separate index and so there may be some delay while site content gets reindexed to power the search.

Some ideas of how to give feedback:
- provide a dashboard in wp-admin
- dashboard in Calypso
- feedback in the front end search UI that content is currently reindexing

Details we could provide:
- estimated time
- estimated number of missing items
- links to dashboards for debugging
- ???

Start by tracing the Instant Search indexing flow and the wp-admin, Calypso, and front-end search surfaces named in the issue. Determine what progress data can be exposed, such as estimated time or missing items; done requires an agreed feedback or dashboard design with debugging links.

Written by the indexing model from the issue text.