Remove openlayers in favor of leaflet

This is big change but I think it will better prepare us for launch. In particular, nearly every map-related thing we're doing becomes simpler.

In particular:

```
 31 files changed, 10200 insertions(+), 86619 deletions(-)
```

It also removes nearly 8% of the javascript we wrote (stuff in `/treemap/js/src`):

```
 3056  8261 96604 total
```

vs

```
 2859  7519 88238 total
```

After

The unminified js bundle jumps down from `3.7MB` to `2.8MB` :crown: