Bug/registration inputs

Registration with missing email or city breaks, Let me know of all the Ruby style guide mistakes I am making =)
I think what we actually want to do here is encapsulate our Signup form logic into a class that implements some of the functionality from `ActiveModel` like we've done for the the `MassMailer` in [`AdminController`](https://github.com/TeaWithStrangers/tws-on-rails/blob/d263c2a2a07a183a5c02c997ba8ce380abfcc19c/app/controllers/admin_controller.rb#L53). Could probably clean up the mess I created quite a bit with some Validator magic!