Delay script loading

[flibbertigibbet]

The site could be faster, particularly on mobile, if script loads were delayed until needed. In particular, map-related things on the home page aren't needed until navigating to get directions, or the screen resizes large enough to see the map. See discussion on #616.

However, there are some interrelated dependencies that may make this challenging. In particular, there is a chain of scripts that expect cartodb.js and Leaflet objects to be defined, but currently need to be loaded whether or not the map has actually been initialized yet. Also, the JS scripts will need to know whether the app is running in development mode or not to know which scripts to load.

So, code splitting and delayed loading via Weback or similar may be desirable, but would likely require significant refactoring.