feat: add simplified MyObservations for logged out users with at least 1 obs

[kueda]

Reviewer notes:

• Only works in debug mode
• Adding obs is in debug mode is current broken pending completion of the new confirmation screen, so to test this PR, you need to add at least one obs while signed out in the default mode, then enter debug mode
• I tried to avoid polluting MyObservations with experimental code, so there's some degree of awkward duplication
• MyObservationsSimple is intentionally monolithic; IMO we can clean and modularize stuff like calculating leaf taxa from the Realm db when we're closer to releasing this; happy to field critiques of style and organization, but I'm mostly interested in whether this meets the product requirements

Closes #MOB-317