Batch API

~~This pull request isn't complete -- it needs more unit tests.~~

That said, I think the JS changes are ready for review. Overview:
* Add a new dict to keep track of which stats data's been retrieved/in the process of retrieval (so we don't try to retrieve it again)
* All requests for stats data (which is currently organized by state/county) get sent to the "batch" api, which handles the responses in one swoop

Python changes overview:
* abstract out reliance on an HTTP request for hmda and census data
* created an endpoint that allows requests for either of these two sources to be bundled together -- this greatly reduces the number of HTTP requests