Show unsynced observations before all others in MyObs

[kueda]

Currently we just sort MyObs by _created_at, which means that any unsynced observations will get buried under newly-created observations on the web or in other apps. Instead,

• show all unsynced observations above other observations
• continue sorting them by _created_at, just show them above all the others
• as they upload they can go back to their previous positions

This will probably involve some weird ordering code.

[kueda]

@budowski this approach works but it's very slow, so I'm temporarily reverting it. When I had 450+ observations this sort operation was taking 1.6-1.8 seconds, making it really slow to load MyObs, navigate to ObsDetail, and come back to MyObs.

Not sure what the better approach might be here. Maybe we need to pre-compute a sort value and store that in Realm?