Prevent abuse of Zenserp API searches

[AllenEllis]

We should have some kind of system to prevent a runaway bot from making requests on the site and depleting the Zenserp API quota.

This could be a multi-pronged approach:

• Rate limit requests from a given IP address
• If the number of requests made to the API exceed a certain number per minute/hour, notify the sysop and/or give people a "try later" message

Particularly because we've cached ~7,000 cities already, API requests should be increasingly uncommon.