* Refactor media sync handling
- The media USN is now returned in sync/meta, which avoids an extra
round-trip.
- Media syncing is now automatically started by the syncing code at
the end of a normal or full sync, which avoids it competing for bandwidth
and resources, and avoids duplicate invalid login messages when the auth
token is invalid.
- Added a new media_sync_progress() method to both check if media is
syncing, and get access to the latest progress.
- Updated the sync log screen to only show the latest line, like AnkiMobile.
- Show media sync errors in a pop-up, so they don't get missed. Use a non-modal
pop-up to avoid potential conflicts with other modals.
* Remove print statement
