Added UUID cols for observations and observation photos.

This is an attempt to ensure posting observations in poor network
conditions doesn't result in duplications.
Obs detail was again broken due to missing obs field values.

We delete blank obs field values on obs save, but that was causing
some integrity issues with the array we use to store them in the obs
detail controller. If you don't delete them, there's this weird
RestKit-based inf loop bug when downloading existing observations that
I don't entirely understand. Kind of patching around it here by only
deleting ones for synced observations.
Changed error message when trying to refresh obs w/o internet.
Made sure sync notice shows when there are things to delete.