Verification challenge code refactor

it works good enough,

future commits will do some refactors to make the success screen more user friendly.

modified:   README.rst
modified:   index.html
modified:   remarkbox/lib/mail.py
modified:   remarkbox/lib/mail_messages.py
modified:   remarkbox/models/meta.py
modified:   remarkbox/models/user.py
modified:   remarkbox/routes.py
modified:   remarkbox/templates/join-or-log-in.j2
modified:   remarkbox/templates/show-node.j2
new file:   remarkbox/templates/verification-challenge.j2
modified:   remarkbox/views/__init__.py
modified:   remarkbox/views/authentication.py