Corrected comments on OpenID temporal fix

Per update from CS50 team, their client will break if not given addresses.

Temporarily return username also as the email and fullname fields of
the response using OpenID's simple registration extension.

Also renamed some variables to make them more distinguishable

Replaced the previous method of getting it from HTTP_POST to use
django's mechanism, which may or may not use HTTP_HOST. However if an
attacker changes the request header, there is not much he can do since
he cannot recreate the association nonce.

missed instance of login link which should have been behind DISABLE_LOGIN_BUTTON

Added OPENID_PROVIDER_TRUSTED_ROOTS django setting. It should be a
list of glob matching patterns: ['*.cs50.net', '*.other.net']

Also did some minor refactoring and cleanup.

- provider placed behind separate feature flag
- domain of openid consumer displayed in login page
- added logging for successful login, changed log messages to include
  "OpenID"

- endpoint supports both SReg and AX
- identity taken from edX username
- sreg fullname and ax http://axschema.org/namePerson taken from edX
  name
- sreg email and ax http://axschema.org/contact/email taken from edX
  email

Use the university profile page for courses and index if we are in a branded subdomain. N.B. THIS DOESN'T WORK WITH MIT_SSL_AUTH