Add browse page

A few components here:

- the actual browse page
- a new model to contain a summary for each address, which is viewed on the browse page
- a script to populate the db with the above summaries (probably run once/day by cron job in production)
- tests!
Lines 59 through 95 could be refactored to loop through once for fire_counts and once for police_counts, no? We probably won't actually have cause to expand it further, but it might be nice to build it more flexibly.