Evaluate Tangram + vector tile basemap

[shua123]

Evaluate using Tangram with vector tiles for new basemap.

Dev version at https://smartchicago.github.io/trailsy/

Needed customization:

• This currently needs a manual build of the Tangram js master branch since it contains a bug fix that hasn't made it to a released version yet.
• This version modifies the leaflet.zoomcss.js plug in to round zoom levels to the nearest whole number since Tangram zooming does partial zoom levels. https://github.com/smartchicago/trailsy/blob/cc-imagery/js/vendor/leaflet.zoomcss.js#L30

Issues:

1. Tile loading seems slow. I think this is because of the nextzen server it is pulling from right now.
2. Its still sometimes not re-positioning items on zoom/movement.

[versatilehuman]

@shua123 Didn't find anything new testing it.

Check out the notes under "Internal" at the end of this release and see if any of those adjustments re: Leaflet are relevant:
https://github.com/tangrams/tangram/releases/tag/v0.14.0

[shua123]

I found an undocumented Tangram option that I think has fixed our main zoom icon out of place issues. (https://github.com/smartchicago/trailsy/blob/gh-pages/js/map.js#L163)

Try it out now @versatilehuman

[shua123]

We moved this to production. Performance still isn't ideal so this may evolve into a new solution.

[versatilehuman]

Time to revisit.

[versatilehuman]

Moved to #278

[gisdevcc]

Closing because we are moving to ESRI vector tile basemaps with esri-leaflet