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.

This path does not seem to make any sense and seems like if would generate an infinite loop.

Just generate a username for them first, by stripping things as you've done above, but don't add the digit. Then, if it's not unique add a digit and check again, etc. Chances are most users won't encounter a duplicate from the beginning.