Support choosing existing (coordinate-less) locations during Birdlasser imports

Existing locations that have lat-long can be chosen by proximity - but anything that doesn't have a lat-long cannot be.

Simplest fix I can imagine is just tweaking the "Choose a name" field to support searching across existing entries. It's already the case that if you type an exact match for an existing location, it chooses it.

For bonus points: take the lat-long of the current entry and set on the associated location.