Visualization of alternate geometries, links to raw alt geom files

[mfogel]

Addresses #119

I went ahead an implemented a select widget under the map. It only appears if the record has alternate geometries. Here's what it looks like on a record with alternates:

Upon changing geometries with the select widget:

• the parent record's polygon is untouched
• the bounding box and consensus geometry polygons are removed
• the markers for the label centroid and center-of-mass are removed
• new polygons for the alternate geometry's bounding box and geometry are drawn
• the map zooms and centers itself to match the alternate geom and bounding box

I wasn't sure where exactly to put PR's for changes to two files: mapzen.whosonfirst.enmapify.js and mapzen.whosonfirst.leaflet.js. Both these files are present in whosonfirst/js-mapzen-whosonfirst, but only enmapify is pulled in by the Makefile. It appears there's been independent changes in both repos of these files so they're no longer in sync. As such, I've just put the changes here to start but let me know if should open up another PR on whosonfirst/js-mapzen-whosonfirst.

This PR also pulls in https://github.com/whosonfirst/js-slippymap-crosshairs/commit/bb40b2b814ae126d8bccc717f641dfed26e93c09 as a consequence of updating slippymap-crosshairs to the latest in it's repo, these changes are technically outside the scope of this PR but AFIAK don't do any harm.

[mfogel]

I'm going to close this PR and open two smaller PR's in it's place - one for the links to the raw files and the other for the map visualizations of the alternate geometries. I think it'll be easier to review the two smaller PR's separately rather than this one big one.