[WIP] Allow developers to upload sources of an Addon version (bug 540028)

Do not review yet (but comments always welcome ;) ).

Ref: https://bugzilla.mozilla.org/show_bug.cgi?id=540028

# TODO

- [x] restrict source file access
- [x] change file link in form widget (or do we want another widget than FileField?)
- [x] add field to "Upload New Version" form
- [x] add field to "Submit a new addon" form
- [x] add field to "Upload a new file" form
- [x] add download link in the "editor page"
- [x] automatically flag addon for 'admin review' when a source is added
- [x] style forms
I propose to rename it `clean_extension` otherwise it's confusing with the standard `clean_source` of the form.