point to thumbnail server, yeoman webapp from django

This pull request has three main changes

 * adds a `requirements.txt` for setup with `pip install -r requirements.txt`
 * adds setting and context processor for `THUMBNAIL_BASE` URL.  `python thumbnail.py` from [md5s3stash](https://github.com/tingletech/md5s3stash) must be running
 * migrates static files from django dirs and `vendor` to yeoman-webapp/`apps`, change `STATIC_URL` to point to `grunt serve`

includes changes from PRs https://github.com/amywieliczka/public_interface/pull/1 and https://github.com/amywieliczka/public_interface/pull/3