Bug 12023 - move InstallAuth.pm to be in C4

InstallAuth was in the installer directory, which meant that Plack was
unable to find it, and so running the webinstaller would fail. This
moves it into C4,

Test plan:
* Make sure the web installer runs under plack
* Make sure the web installer runs under non-plack

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 12023: (follow-up) fix redirect to step 3

This patch fixes the redirection to step 3.

If you go on installer/install.pl and Koha is already installed,
you should be redirected to the step 3. Without this patch, the
script raised an internal error (500).

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Robin Sheat <robin@catalyst.net.nz>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>