Batch API changes

Several changes here, but nothing crazy.

* Modify both HMDA and census data endpoints to accept (and query based on) multiple counties
* Modify both HMDA and census data endpoints to accept counties as a 5-digit state+county fips code
* Modify batch endpoint to use GET rather than POST requests, removing much unneeded schema-related code
* Modify JS to use this system rather than POSTs
* Move batch/conversions to batch/utils
* Remove census "binning" endpoint -- it's never used
* Fix presumed typos in JS referring to a non-existent variable
* PEP8 and JS Lint fixes

Benefits are around load time. By allowing multiple counties, we limit the number of database calls. By using GET, we allow the browser to cache the results.