Bug 643: QA Followup - server-side verification of permissions

Bug 643: QA Followup - server-side verification of permissions

Only allow $force_allow=1 if the logged in user has permissions.

TEST PLAN
---------
Attempt to intentionally override the checkout by passing an
appropriately handcrafted URL.
-- Regardless of the force_allow value, it should be not allowed
   for those lacking the force_checkout permission.

NOTE: I didn't test this. I figured Marc Veron could do that. :)
      (Sorry, couldn't easily get git bz to work with the accent)

Signed-off-by: Nick Clemens <nick@quecheelibrary.org>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>