Several fixes to RegistrationType handling in Two-Phase Student Reg

1. Use `reg_type.__unicode__()` rather than `reg_type.displayName`
directly (in case `.displayName` is not set, we should default to `.name`,
which `.__unicode__()` already does).

2. Use `RegistrationType.get_or_create` in /rank_classes view, so that
the page doesn't crash if one of the expected RegistrationTypes
doesn't exist yet. (Also make the variable naming more consistent.)

3. Don't use `get_or_create` in /save_priorities view. Previously this
was a minor security hole that allowed students to e.g. register for a
class with a RegistrationType of Priority/{arbitrary injected string}
by submitting tampered post data.

4. Fix a jQuery selector so that the page doesn't try to submit empty
registrations with Priority/undefined.