Adding support for customized avatar services.

This new feature allows the user avatars to be fetched from any other service, based on any of the users details, such as the username.
The lookup is performed by email, to load the user record, and then a given field is extracted from the user record to be substituted on the avatar image URL format string.
Shibboleth integration and omniauth/LDAP code clean up.

Configuration format now handles arbitrary omniauth modules.
LDAP is treated just like Shibboleth and any other omniauth authentication method.
With Shibboleth, the login screen in Gitlab is hidden and session is started automatically because the user can only access the site after successful login on Single Sign On.
These changes require gem omniauth-shibboleth 1.0.7 or later.
Bug fix - Missing 'end'.