[WIP] migrating to staticfiles

See https://bugzilla.mozilla.org/show_bug.cgi?id=1013865
Not ready for review yet.

Overview:
- `STATIC_URL` and `MEDIA_URL` have been split (`STATIC_URL` default path will be `/static/`, default `MEDIA_URL` will be `/storage`: we have chose not to use `/media/` to avoid confusion, and to make easier to manage redirects after first push)
- `STATIC_ROOT` and `MEDIA_ROOT` have been split
- static files have been moved to /static instead of /media
- some helpers have been created to make that is possible to override `MEDIA_ROOT` locally without needing to override all the `_PATH` settings

Maybe best files to review first are `apps/amo/helpers.py` and `lib/settings_base.py`.
For the record, because moving git submodules is painful, we might need to do https://bugzilla.mozilla.org/show_bug.cgi?id=1035783 first