Introduce trusted omniauth

Trusted auth is implementation to group all different strong or trusted authentication methods under same code and policy.

Trusted authentications for now are ldap, pam, cas(?) and shibboleth(?). From these ldap and pam works similarly. They ask username and password for authentication. So to implement shibboleth, some modifications are required.
Move ldap specifics to trusted omniauth interface
Add has_trusted_authentication? to user