Update contributors.
Crashlytics update.
Migrating favorites relies less on AwfulSettings.

We can use the migration manager's userInfo dictionary instead.

Originally I'd tried to store an ivar in the migration policy, but new
instances of the policy get created as the migration continues.
Extract AwfulErrorDomain and codes.
Fix favorite migration.

I had no idea what I was doing before :-( Now we correctly migrate the data
store up to the version just before favorites get migrated, then run the
favorites migration. (Of course we only do this if we haven't already migrated favorites.)