Multilingual templates

This is my proposal for multilingual templates. Templates can be created with an optional language set, and the EmailTemplate loader will try to load the template for the currently active language. 

I'm not sure what to do with the order of the migrations. Also, there is one caveat with caching; if you change the language of an EmailTemplate, the original cache key should be invalidated. But you don't know the original language in `EmailTemplate.save(self, ...)`. Anyone any ideas how to solve that?