Statistics and metadata scanning progress page

[DutchJaFO]

It was mentioned on Discord that there is a metadata refresh interval set to once every 90 days for artists, albums and tracks, and once every 7 days for playlists.
This was done a while back when we started getting requests from musicbrainz to limit our lookups.

As all of this done in the background there is no way for the users to know if any of this is happening at all.
Some of that info may be gleaned from logging info, but that only happens when there's critical issues and few users would look at logging info under normal circumstances.

This feature should satisfy the following :

• overall statistics of the media library (total tracks, albums, artists, etc).
  This info is already available when viewing the pages, but could be listed per provider (if/when available).

• time of last update & time until next planned update
  This allows users to verify the scanning is actually happening. Future iterations could include the option to manually trigger a scan.
  A related feature to 'synchronize now' is available in the provider-settings. However some users may prefer to have features like this available to users who shouldn't have access to the settings themselves for various reasons.

• additional performance related statistics would be nice to have (we already have some of these in the logging as provider scans log the time needed to complete)

Maybe expose some of this info as 'sensors' for HA ?
The PLEX integration does something like this for HA.