Improve city selection

[ericharmeling]

We should update the server logic to use "New York" when the client is in an unsupported city. The server should alert the user that their city is not yet supported, and that they could experience much higher latencies as a result.

• (Quick fix) Set city to "New York" for all requests coming from unsupported cities.
• Add region selection.
• Allow any city.

[irfansharif]

Copying over my comment from slack: it would be cool to let the user just pick where their requests are routed through, so they can experience the varying degrees of painful latencies.

[ericharmeling]

#8 does not actually resolve this completely. Reopening for now.