Localize Agreements

Fixes #158. This adds a new model to allow associating additional PDFs of the agreement for non-default languages.
It seems to me like this would read better defined outside of the class -- the usage of `instance` would make more immediate sense, all the class attribute definitions could be grouped together, and also the way it is used later in [line 47](https://github.com/mozilla/fxoss/pull/171/files#diff-f30618d89fa9b40f946c6e5dd8c5a2d6R47) as a function instead of a method.