Migrating to staticfiles

See https://bugzilla.mozilla.org/show_bug.cgi?id=1013865

Plan is to land this to master/dev just after next push (tomorrow).

## 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
- we will need to decide of a `STATIC_ROOT` for each of our servers (dev, stage, prod)
- we will need to add `collectstatic` command to our deploy process (already added in `fabfile.py` and `update.py`)
- we will need to update our Nginx config to server statics from this `STATIC_ROOT` folder
- versionned static files have been moved to /static instead of /media (but we don't serve them from here in stage/prod)
- we will need to handle redirect from /media/ to /statics
- `MEDIA_ROOT` is the new `UPLOADS_PATH`
- we will need to handle redirect from /img/uploads/* to /storage/*
- persona media are a special case: they were served from a `LOCAL_MIRROR_URL`, and they will be served as normal medias
- all the media related `XXX_PATH` settings have been removed, but can still be used to override default storage path where neede; instead we use the helper `storage_path`
- all the media related `XXX_URL` settings have been removed, but can still be used to override default URL where needed; in the code, we now use the helper `media_url` instead
- the django-moz-header submodules have been also moved during the `media` to `static` move, which make the git game a bit harder, there is a [discussion](https://bugzilla.mozilla.org/show_bug.cgi?id=1035783) about moving them to a real django app
- jingo-minify has been [patched](https://github.com/jsocol/jingo-minify/pull/64/files) (and the plan is to move over from it, see https://bugzilla.mozilla.org/show_bug.cgi?id=1024116)

## Some examples of URLs

| Before | After | Comment |
| ------------- | ----------- | ---- |
| https://addons.cdn.mozilla.net/media/img/zamboni/discovery_pane/promos/football-worldcup-2014.jpg?bf0901d | https://addons.cdn.mozilla.net/static/img/zamboni/discovery_pane/promos/football-worldcup-2014.jpg?bf0901d | static URL |
| https://addons.cdn.mozilla.net/img/uploads/addon_icons/2/2324-32.png?modified=1394059891     | https://addons.cdn.mozilla.net/storage/addon_icons/2/2324-32.png?modified=1394059891     | media URL |
| https://addons.cdn.mozilla.net/_files/523928/header.png?1403198428 | https://addons.cdn.mozilla.net/storage/addons/523928/header.png?1403198428 | specific persona URL change |