Refactor PurpleAir sync process to remove intermediate tables

[ianhoffman]

Now that all metrics we need to serve queries and send alerts are stored in the zipcodes and clients tables, we could do away with the sensors and sensors_zipcodes tables entirely. We'd need to build the mappings between sensors and zipcodes in-memory every time we sync, which might be slow (it's a lot of geohashing), but it's definitely doable.

This is just an avenue for exploration.