Datamodel and performance improvements
- Total comment and unread comment count per PR is now recalculated and not done on each query.  This also allows filtering out of PRs without comments at the DB level, which is the only way to do it on iOS anyway.  Side benefit is a large performance gain on OSX too.
- Added nuking of DB in case of incompatible DB schema - app would get caught in an infinite loop until now.
- Corrected iPhone display preference labels and made two int conversions 64-bit safe
- Replaced deprecated text sizing code