Customize form data cleaning

[bepetersn]

This can be done by overriding the clean methods on the Form classes that we have defined. Below are a few ideas.

EventForm:

• Check if sessions invalid times (i.e. outside the time limits of the event that they associate with)

AddVolunteerForm:

• Check that user's name is not empty
• Check that username hasn't been taken

LoginForm:

• Check if user has an associated volunteer (if not you're a superuser, and you shouldn't use vol. login)