Ability to filter resources by state

[alan-fang]

Is your feature request related to a problem? Please describe.
What seems to be the easiest way to distinguish resources geographically is to filter by state. Angela has provided some feedback about providing resources within a specific state ("Hover over a state and see how many resources are available?", "Filter = search by continent/country, state, city?", "Is there a way to click on a state and have resources in just that state pull up?"), so adding functionality here seems prudent, even if we don't end up adding all of this functionality.

Additional context
Probably a couple of ways to accomplish this. We can modify /filter, or add a new endpoint, or something else (up to you!) You'll probably have to play with the Mapquest API, or figure out a way to pull the state from the address field in the Resource model.

[alan-fang]

finish #191 first and then work on this