Bug 16258: Add a syspref to govern EDIFACT functionality

Bug 16258: Add a syspref to govern EDIFACT functionality

Adds an "Edifact" systempreference to govern whether edifact processing
is enabled. In most places this is hidden if the current
vendor does not appear in the edi vendors table. This preference
hides the admin screens which define this and a couple of links.

Also fixes an anomaly whereby the basketgroup screen was not
making the same check on whether edi ordering should be enabled as
the basket screen. Both now use the same logic.

Rebased-by: Mark Tompsett <mtompset@hotmail.com>
Rebased-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>