Add an opt-in checkmark

[bmann]

Add a checkbox in small right before the submit button. Something like:

"Yes, I understand that I'm opting in to sign up for the Fission product email list. Send me stickers!"

[matheus23]

There's a checkmark now: I've improvised the style a little. Hope it looks alright.

Its position and text are configurable in the .yml, of course.