Project content update

Updates Project Content to Match what Brian posted here: https://github.com/zooniverse/Panoptes-Front-End/blob/master/app/data/projects.coffee. 

Also adds an endpoint for adding and updating project content translations. I'll add versioning after #213 is accepted. 
I think it makes sense to treat the canonical language strings as part of the main resource, and provide a different endpoint for updating translations. It also simplifies access control a little bit since we don't want translators updaing the primary language content only project owners/collaborators.