Bug 26549: Show value of global system preferences on subscription form

Bug 26549: Show value of global system preferences on subscription form

This patch modifies the subscription entry form so that the values of
two system preferences are displayed as a hint to the user:

The "Number of issues to display to staff" field will show the value
of the StaffSerialIssueDisplayCount preference, and the "Number of
issues to display to the public" field will show
OPACSerialIssueDisplayCount. If the user has permission the system
preference names will link to the system preferences page.

To test, apply the patch and log into the staff interface as a user with
permission to edit system preferences.

- Go to Serials -> New subscription.
- The "Number of issues to display to staff" field should have a hint
  under it: "Default: XX (StaffSerialIssueDisplayCount system
   preference)"
- The "Number of issues to display to the public" field should have a
  hint under it: "Default: XX (OPACSerialIssueDisplayCount system
  preference)"
- Log out and log back in as a user who does not have permission to edit
  system preferences. Return to the form and confirm that the hints do
  not link to system preferences.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>