When auth.type = HTTP or HTTP_LDAP, allows the configuration of an
explicit login URL in order to trigger the authentication process of
the front-end SSO system.
URL and link name are customizable using:
* auth.loginUrl
* auth.loginText
Configuring a server with auth.loginUrl allows an unknown user not yet
logged in to perform anonymous browsing of Gerrit, as allowed by other
auth.type methods.
Change-Id: I52aa7950fdf0ba23a55a7d4eb5f1f1e3f6be6b38
5185b040d3