Split exams app in half

The exams app (now renamed `schedule`) now only takes care of showing
upcoming exam sessions and their locations.

The prereg module (now upgraded to a full Django app) now takes care
of storing registration information (more so once I actually let users
register for sessions).

Undecided on whether prereg.models.Examination should stay there or
move to a different app.
Miscellaneous PEP 8 fixes