Revise localized agreements

This is a very rough draft of revising the way we're localizing the agreements after the discussion following https://github.com/mozilla/fxoss/issues/199#issuecomment-51694614. Will utilize the "Ready for Review" label when appropriate.
This should probably by in an `Important` block, and the wording is a little awkward. What you're trying to convey is that agreement versions are a per-language setting, and that when an agreement in the current language can't be found, the site falls back to to the earliest-uploaded agreement with a matching version. Putting it that way establishes a simple rule that can be applied, which I think makes the "older versions coexist with newer version" feature more comprehensible.