Introduction of a new auth.registerPageUrl in
gerrit.config to specify a custom registration
page when a new user logs in the first time
to Gerrit.
If not set, the standard Gerrit registration page
`/#/register/` is displayed.
Needed when SSO with a 3rd party authentication
system (e.g. GitHub) is required and the initial
migration of the user profile (e.g. SSH Keys)
would require custom user interactions.
Change-Id: Iad0941e97c0fa749027f12f17f584eb2bd6de07b
111e0b7934