1556 session type

This was harder than I had expected. Mostly because there is so much to consider when changing studies:

• export/import
• duplicating studies
• study type disinterest

So I erred on the side of simplicity where possible:

• The model is still called StudyType
• Study.study_type still mostly works as before
• create_demo_content no longer assigns study types
• Import creates a single session (just like migrate)
• Sessions are not considered when duplicating studies. We might changes that when moving sessions to recruitment settings though.

We could change most of this, but it would be quite some work.