simplify addresses_controller

- we don’t need to return lat/lon;
  since we don’t use PostGIS anymore, we can’t 
  get the center of polygons anyway, and we’ll
  just let the frontend code use the center of the
  geojson district polygon

- remove a lot of unnecessary logic
only pushState if lat/lng came from map

- we only want to pushState when the user selects
  a new location via the map; if we don’t have that,
  we’re looking at a page the user selected through
  a link or form submission and the history entry has
  already been saved by the browser
center map if no lat/lng given
fixup: make sure district exists before testing it
if address was provided, use it in the url
example address link should go through root (/)

- if the user was on a page like /mayor,
  we don’t want to jump to /mayor?address…; instead
  we want to treat the address query the same way
  we do when the go there from the front page.
cleanup addresses_controller: remove commented code

- it was old and unused
config.js: document which is lat., which is lon.
Merge pull request #116 from codeforamerica/bugs/misc-fixes

simplify addresses_controller, highlight selected district, and fix example address link