Bug 17669: Introduce new preference for purging temp uploads

Bug 17669: Introduce new preference for purging temp uploads

The new preference is: Upload_PurgeTemporaryFiles_Days.
It controls if the cron job will delete temporary uploaded files and if so,
how many days should have passed before doing so.

Since the default value is empty, it will not start deleting these uploads
until you change the pref. No behavior change. But the dbrev will print a
message if you already have temporary uploads.

Test plan:
[1] Check Administration, Preferences, Tools for the new pref.
[2] Run the db rev.
    If you have temporary uploads, check the reported count.
[3] Bonus: Run sysprefs.sql in a safe environment.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>