Bug 11759: (QA followup) Sysprefs should be kosher

Bug 11759: (QA followup) Sysprefs should be kosher

The introduced syspref was defined before we added new guidelines
for them. This patch puts them in sync with current standards. It also
splits them into:

 - A big on/off switch (BatchCheckouts)
 - Patron category filter (BatchCheckoutsValidCategories)

The latter is expected to go away if we move this into a boolean column
on the 'categories' table, which seems a better approach. I'm filling a
new bug for this last comment.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>