Integrate proof of concept gulpfile into Django project

 - Changes the Django `static` and `media` directories to be inside `/var/www/nyc-trees/`
 - Adds the `static` template filter for converting static file to use their versioned path
 - Fixes a few issues with the proof of concept `gulpfile`
I'm keeping `VAGRANT_ENV` unset, so right now I'm just testing the development experience. Every time I've tested, I've destroyed the `app` virtual machine and recreated it:

```bash
$ vagrant destroy -f app && vagrant up app
```