Create Obs: Find the best-fitting default Location when none given

[nimmolo]

Create obs defaults to "Earth" when no Location is given and the user declines to create one.

When we have the GPS point, it seems we could at least make location selection easier, and if they decline all selections, default to the smallest appropriate location.

We could offer options from a query for locations that include the given point, ordered by area ascending (i.e., where the bounding box, plus perhaps a delta beyond, includes the point)

Rationale: In some cases it may be that associating the obs with a location seems confusing or unnecessary. Or maybe it seems the Location table is incomplete, lacking many well known places, and people don't want to go to the trouble of creating a new location.