request #5951 - Avoid hardcoded redirect on login when possible

When we configure Tuleap behind a reverse proxy that does URL & domain rewriting
most of the time, it doesn't play well with hardcoded redirect (with the domain
name in the redirect request).

As we only need the domain when we change of protocol (http <-> https), starting
this commit, Tuleap will do relative redirects (without domain name) when there
is no protocol change.

As a matter of fact, when Tuleap is configured in such a setup, it should stick
to one protocol (either full HTTP or full HTTPS) and not allow both.

Change-Id: I789d914b466d573453a6c33c8f7b4783f0c66bcd
Merge commit 'refs/changes/25/1725/7' of http://gerrit.tuleap.net/tuleap into HEAD

* http://gerrit.tuleap.net/tuleap:
  request #5951 - Avoid hardcoded redirect on login when possible

Change-Id: Ie5d1ff29d593fbe0907cd28a4e91f7cdd36cfee7
This is Tuleap 6.10.99.18

Change-Id: Ib576a228a34b347828e18cf334f45380eaa75a3b