Store imported google contacts in the contacts db

This PR stores the Google contacts in the indexeddb and allows them to be cached across restarts. This is intended as the first step for storing contacts.

This PR:
- Upgrades the database to a v2
- Adds an index for "source" (although this isn't used directly at the moment, due to [bug 935457](https://bugzilla.mozilla.org/show_bug.cgi?id=935457), I think the code is worth keeping)
- Adds a batch update for the database. Although this could be better constructed, it is likely to mutate in future, and be made more appropriate for sync.