Bug 14004: Add ability to temporarily disable JS/CSS sysprefs

Bug 14004: Add ability to temporarily disable JS/CSS sysprefs

It would be *super* handy if intranetuserjs and/or opacuserjs could be
temporarily disabled via a check-box or syspref.
Right now, debugging issues in intranetuserjs usuaally starts with
copying the contents into a text file, blanking the syspref and re-testing.

This patch adds this feature by setting syspref via ENV
OVERRIDE_SYSPREF like override via Apache config.

Implemented only for preferences :
OPACUserCSS OPACUserJS IntranetUserCSS IntranetUserJS
=> replaced with ' '
intranetcolorstylesheet intranetstylesheet
=> replaced with 0

Test plan :
1) Set some CSS in IntranetUserCSS like : #breadcrumbs{color:red}
2) Go to staff interface home page like : /cgi-bin/koha/mainpage.pl
3) See CSS impact is visible
4) Edit URL : /cgi-bin/koha/mainpage.pl?DISABLE_SYSPREF_IntranetUserCSS=1
5) See CSS impact is not visible
6) Check with the other preferences

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>