[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
- [ ] 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
> Remove the ellipsis if only these 2 extensions are tested.

I think we will add more before pushing this :)

> BTW, should we try to unarchive it to verify that it's a valid archive?

I thought about this, but I think it's not really needed. Those archive are uploaded by a developer to allow the reviewers to validate its add-on. If the developer uploaded bullshit, it will not get its add-on reviewed, and then he will not do that.
Now I don't think there is any security issue, but if there is any, I would give for sure another answer ;)