run stripe form generation through django

this is so we sanitize inputs automatically, which we should be doing here but weren't. security thing

@EliAndrewC  could you look this one over very carefully.  I tested it out a bunch and it looks like it should be sane for everything.

We should make a mental note to test all the various ways in which we do Stripe forms in ubersystem to make sure this works there.

I think this fixes #283
Since the empty string is a falsey value, it's more idiomatic to just say ``if email`` rather than ``if email != ''``