Add logic for importing walk-in class from previous program

Re: https://github.com/learning-unlimited/ESP-Website/issues/728

Checks for class category on copyaclass with POSTed request to route to correct action and allow importing of walk-ins.
Also, I realized you don't actually need to get the category from the DB here; you can just check if the POSTed category ID is the same as `self.program.open_class_category.id`.  Not a big deal since it's a single `.get()`, but if you get a chance you might as well fix it.  This might also make the code even simpler if you're going with the error approach I suggested.