Project content

This adds a ProjectContent model that holds the global content for a project 'pages', 'example_strings', 'title', 'description'. It also adds a way for a user to set languages preferences as well as a way of determining the best language to respond to a given request using the priority order is (lowest to highest): 

1. Query Param
2. User Preferences
3. Accept-Language Header
4. Project's default language. 

We will probably need a similar thing for Workflows to hold translations of task questions and answers.