Use Nominatim for location tagging

[d4f5409d]

Nominatim search for OpenStreetMap data is more reliable.

https://nominatim.org/

[Should fix #2]

[realpixelcode]

That's not a good idea, because Nominatim's usage policy explicitly forbids auto-complete search. Also, there's a maximum of 1 request per second per app. That means globally, only one Pixelfed user could make use of the Nominatim integration per second.

https://operations.osmfoundation.org/policies/nominatim/