admin csv export improvements

- rename original csv to pledge_amounts.csv
- create new csv export for the histogram called pledges.csv
- use zipgun library to convert zipcodes to lat/lon
- change csv generation queue rate limit to 1/h
- provide persistent link to latest export

@jwhite66 If you want to push this to staging so we can test, we could explore the possibility of using this new export to update the data for https://github.com/MayOneUS/homepage_redesign/issues/98