Make vendor.neighborood editable

Prevously neighborhoods were automatically assigned and not editable.
Now, the neighborhood can be edited. We'll see if this solves problems
or creates them.

There's a 'gotcha' here. If you add a vendor and then customize the neighborhood,
it'll get clobbered if you ever change the address.

So, I add "Soy Cafe" and it says "North Philadelphia". I change it to "Northern Liberties"
and everyone is happy. Then, I realize that I got the address wrong. I change it, and now
the neighborhood is "North Philadelphia" again.

To avoid this would involve writing a lot more code. I think this is a suitable trade-off for
v1.0, as it can't make things any worse than they are now, despite being ambigious.

Assigning to @jfarbowitz to review the behavior change at a high level.