Bug 1006569 - [privacy] Add checkbox at sign-up

Functionally works fine for me and all the tests pass too.
I will add the unit tests and modify helper text later today and need to discuss about the deletion of user email from database usecase.

Although it isn't in the wireframe, I think we need a colon after the label and before the actual form field. That would match the Task form I created, and currently the `Username` field looks confusing on the form the way the field name (Username) and then the prefix (https://oneanddone.mozilla.org/en-US/profile/) are squished together. We should add the colon between the word `Username` and the prefix in that example.