Sef 2459 exam edit command

We needed a utility to change dates on exams.  I got a bee in my bonnet to do something more general, hence this command.  It's actually really dangerous ("set slug=foo for all exams") but I didn't want to bother with a bunch of special-cases since this should be so rarely used, and it defaults to a dry-run mode (a la regrade).

@jrbl or @jbau please have a look.  If it passes muster then I'll do a prod branch so we can hotfix and use for dbclass.