OSM

I've taken #39 and updated it with the current master. This integrates all of @yohanboniface's work for switching from GMaps to Leaflet with tiles from thunderforest.com.

Additionally, I tried to simplify the template blocks inheritance chain, using `data-` attributes instead of inline javascript for altering map options.

TODO:

- [x] Tiles over HTTPS: `https://djangopeople-tiles-{s}.herokuapp.com/transport/{z}/{x}/{y}.png`
- [x] JS cleanup: we should have a proper JS library with no global variables. `common.js`, `maps.js` and `signup.js` should be merged.
- [ ] Move remaining inline JS code to said library via external scripts.

I've added Yoann as a collab on the repo, let's work on this in the coming days -- this isn't ready for merge yet :). But having a repo with collabs is much easier for collaboration than dealing with pull requests from forks.