Bug 13496: make auth.tt use a template plugin to get the Koha version

Bug 13496: make auth.tt use a template plugin to get the Koha version

This patch makes the installer login template use the new Koha.Version
template plugin to get the correct Koha version. This way we don't need
to maintain the auth.tt file on each version change, and are sure screen
readers tell the correct version.

To test:
- Create a situation where the login screen from the installer shows
- Navigate the HTML code (Ctrl+u)
=> FAIL: "Koha 3.0 Installer" shows on the HTML code
- Apply the patch
- Reload the page
=> SUCCESS: The correct version number is shown.
- Sign off :-D

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>