Improve design and behavior of Census tract maps

[migurski]

Two maps showing demographic survey responses could use a lot of improvement.

Here’s a live example: survey-page-results.html (watch the http vs. https URLs, Chrome tries to SSL you but it needs to be http).

Needs

The maps do a good job of showing the basic information, but could use a bit of improvement:

• [x] Switch to non-retina tiles on non-retina screens.
• [x] Possibly switch to separate foreground/background layers, e.g. toner-background and toner-labels. Is this even possible?
• [x] Design popup content with more than just a link to a Census Reporter page.
• [x] Improve visual appearance of demographic buttons on right-hand map.

  Code

Relevant files and functions:

• DemographicsControl in lib.js
• build_map() in lib.js
• Layer onEachFeature in lib.js

Depends on #4.