Add grunt watch task.

Rebuilding the JS is still slow, but now it will auto rebuild when a file changes by running `grunt  watch`, locally.
I think we should rethink rework the names a bit, to avoid publicly exposing `static` and `collectstatic`, which are too similar.

Here's some ideas, not necessarily great:

* `bundle` became `bundle_js` or `_bundle_js`, or (don't know if it's used my jobs)
* `static` became `bundle` or `full_bundle`