Cleanup of useDraftObservation() hook

[gmaclennan]

Managing draft observations is a critical part of the app, and a part that is really important to get right. Currently the logic for managing a draft observation is split between two hooks useDraftObservation and usePersistedDraftObservation. Some properties from one hook are the same as the other, and some are different. We also return a hook from a hook, which is confusing.

Cleaning up and simplifying this code should make it easier to maintain this part of the app and to write unit tests for the important logic around updating presets and saving photos and audio.