Move some model logic out of a presenter; tidy up test output.

Here's some changes, as discussed earlier. I think writing tests for these might be a bit unwieldy, but I guess we'll see how much Coveralls complains. 

The basic idea is that the "type" of invite (student/coach) is an attribute of the invite and whether there's room for a student/coach is an attribute of the session, so it moves that logic out of view helpers and down into the model. 

There's also some small changes on the Rake tasks, to make the test output neater.