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.

The form does not match the wireframe [1]. Please update it so it matches.

[1] https://bug1006569.bugzilla.mozilla.org/attachment.cgi?id=8444772