Reorganizes Sync Manager and moves comments to Sync Manager. Also, adds last fetch times to all models.

What this does
---
 - Logically reorganizes sync manager.
 - Moves fetch times to respective parent models.
 - Move downloading comments to sync manager. 
 - Syncs comments in the background.
 - Adds numpad return key to the add comment submit function.
 - Makes Date formatters more thread safe.

cc @joshpc @j-mutter