Localize Agreements

Fixes #158. This adds a new model to allow associating additional PDFs of the agreement for non-default languages.
This is much more readable, thanks, but I just notice a small typo: would you mind adding an 's' to 'tranlated'? :)
Also, as long as you're doing a search and replace for the function name, you might consider a shorter alternative such as `get_translated_filename` or even `translated_filename`