Bug 29744: Harmonize psgi/plack detection methods

Bug 29744: Harmonize psgi/plack detection methods

This patch updates and moves the existing psgi_env method out of Auth
and into Context and then replaces any manual references of the same
code to use the new method.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>