A /force_auth screen.

* Save forceAuth to the session. If forceAuth is set, only show back button in the reset_password screen.
* Reset the forceAuth flag from sign_in and sign_up
Require an email address to use the /force_auth screen.
Merge pull request #335 from mozilla/issue-334-force-reauthenticate

/force_auth?email=<url_encoded_email> screen.