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

From Jorgev:

- [x] In the review page, the Download Source option should only be available to Admin Reviewers (group 50005). I would also use "Additional sources" so it's clearly distinguishable from the sources in the XPI.
- [x] In the upload forms, the source code form should appear after the upload and validation for the XPI succeeds. Otherwise it might confuse developers.
- [x] I would change the label "If your add-on contains binary or obfuscated code other than known libraries, upload its sources for review. Read more about the source code review policy." The 'Read more' text should link to https://addons.mozilla.org/developers/docs/policies/reviews#section-binary and open in a new tab.