allow advertising gitblit privileges for external URLs

commit c20191fc0931a19bec0df1ab2b56f287e5d8b7c7 enabled support
for hiding internal URLs, but didn't consider that it broke the
evaluation of permissions (used for tickets, etc.), and caused
a NPE on repoUrl.permission when trying to view the TicketPage.

With all internal mechanisms disabled, it would result in the
first URL being external with unknown permissions. This adds an
option to use internal permissions even for external URLs.

Note that this does not grant any additional permissions, but
does offer the option to have gitblit advertise the full set of
what is allowed, even if the external URL imposes additional
restrictions.