OpenSAML integration, based on shibboleth by TiagoTT

* Allow SAML OmniAuth users to login, just like LDAP users.
* Unifying Omniauth configuration.
* Adding automatic SAML login if SAML is enabled.
* Hiding Omniauth and Password sections in Profile/Account settings when omniauth.allow_single_sign_on is enabled.
* Hiding GIT HTTP URL on project homepage if SAML and Single Sign On are in use.
* Add SSO metadata.