Bug 12539: PROG/CCSR deprecation: Remove hardcoded theme from C4/Templates.pm

Bug 12539: PROG/CCSR deprecation: Remove hardcoded theme from C4/Templates.pm

This patch removes hardcoded 'prog' from themelanguage on C4/Templates.pm
It adds a new syspref OPACFallback to hold the prefferred fallback
theme for new themes with minimal changes.

This does not brake CCSR if new syspref value is 'prog',
using 'bootstrap' works somehow, but with strange results.

To test:
1) Apply the patch
2) Run updatedatbase.pl
3) CCSR, prog & bootstrap must work as usual
4) Need to test theme/language fallback
 a) translate for your preferred language (xx-YY)
 b) enable language on opac
 c) go to opac, select that language
 d) login on opac, go to personal details
 e) remove koha-tmpl/opac-tmpl/bootstrap/xx-YY/modules/opac-memberentry.tt
 d) reload page, must load in English
 f) click other member options, must load on translated language
(can use other pages, ccsr, prog or bootstrap)
5) Do a similar test but for a staff page, enable lang, switch to it,
rm koha-tmpl/intranet-tmpl/prog/xx-YY/modules/admin/admin-home.tt,
any page must be translated except for admin home.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Petter Goksoyr Asen <boutrosboutrosboutros@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>