feature: Improve 2FA

[jwag956]

A rather large refactor with 2 main goals:

1. remove sending personal info as part of initialization during signup
2. implement opt-in in addition to 2FA required mode.

While the basic flow and forms didn't change - there were many changes:

• A new configuration variable SECURITY_TWO_FACTOR_REQUIRED (default False)
  If you want old behavior of requiring 2FA - this must be set.
• Contents of session cookie are completely different.
• CHANGED: Signal names:
  • 'user-two-factored' -> 'tf_code_confirmed'
  • 'two_factor_method_changed' -> 'tf_profile_changed'
• 2 New signals introduced: 'tf_disabled' and 'tf_security_token_sent'
• Code for most 2FA views changed dramatically, however the actual flow
  should be compatible.
• CHANGE: if call /tf-setup and haven't re-confirmed password - now redirect to
  two_factor_confirm_url rather than login_url.
• CHANGED: as part of naming rationalization - the context processor names changed:
  • two_factor_change_method_password_confirmation_context_processor -> tf_password_verify_context_processor
  • two_factor_setup_context_processor -> tf_setup_context_processor
  • two_factor_token_validation_context_processor -> tf_token_validation_context_processor

Various bugs and doc improvements:

• The two factor _VALIDITY configuration variables were fixed to reflect prior changes
  which meant these values are now in seconds.
• A new message TWO_FACTOR_DISABLED was introduced

Testing Improvements:

• view_scaffold.py was introduced that makes it easy to test forms. This is a real
  Flask application that can be run, and using a normal browser can interact with
  various workflows.

Closes #95

[TillerBurr]

The views did change quite a bit, glad I didn't start writing tests. I think this is good to merge.

[jwag956]

@baurt thanks for your review. I incorporated your ideas. Will merge after that.
Just one more refactor to do - as laid out in issue #102

thanks for sticking with this and hopefully this isn't causing you too much work.
With 102 in I will cut an rc2 and see if you and maybe some other can try it out - then release it - hopefully by end of june.

[TillerBurr]

@jwag956 No worries. I enjoy looking at it and helping improve the library.