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.
![screen shot 2014-11-02 at 8 42 48 pm](https://cloud.githubusercontent.com/assets/2312335/4877895/c4bf7266-62fa-11e4-8add-ea1641a4e585.png)

I'm mimicking other errors a few lines down, if there's something else I should be doing I can change it (or make the error message more detailed, or whatever)