Investigate offering selection when submitting

In ivle.webapp.submit.UserSubmitView, the get_offering method is a bit confusing. Here it is:

        return Store.of(self.context).find(Offering,
            Offering.id == Enrolment.offering_id,
            Enrolment.user_id == self.context.id,
            Offering.semester_id == Semester.id,
            Semester.state == u'current',
            Offering.subject_id == Subject.id,
            Subject.short_name == self.path.split('/')[1],
            ).one()

That means, find the offering as follows:
- Find the subject S with the short name matching the second-level directory in the user's repo (FAIL if the user is not in a directory named after a known subject),
- Find all offerings O for subject S,
- Drop any offering which is not a) in a "current" semester, AND b) the user is not enrolled in,
- Fail if the offering set O is empty. Otherwise, take one offering, and return it.

There are a few problems with this:
- Many failure conditions (not in a subject directory, user not enrolled, or user enrolled in an offering which is not current). With the fix for bug #526853, we handle this by giving a vague error message. We should detect exactly why it failed.
- Possibility of multiple offerings matching the query. If there are two offerings of the same subject which are both in a "current" semester for some reason, then it is undefined which offering is being submitted to. I don't know what .one() does if there are multiple results (error, or just pick one?). We should figure out a strategy for handling this case.