Bug 12023 - move InstallAuth.pm to be in C4

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>