Add a new tree-structured Section model, based on MPTT
Move the definition of Section to earlier in models.py

It's required by Speech, and it's slightly nicer to refer to it
with a class object rather than a string, I think.
Add a 'section' field to Speech
Add the instance to sections, and update the tests
Data migration to sections.
Allow returning of sections ordered by speech times within them.
Update front end to use sections, and remove debates/meetings.