Bug 25919: Only show Desks in admin navigation when UseCirculationDesks is used

Bug 25919: Only show Desks in admin navigation when UseCirculationDesks is used

We were missing a check for the UseCirculationDesks in the
template so the "Desks" link would only appear in the admin navigation
when the pref feature is activated.

To test:
- Turn off UseCirculationDesks
- Go to any administration page that shows the navigation on the left
- Verify Desks shows
- Apply patch
- Reload the admin page - Desks should be gone
- Turn on UseCirculationDesks
- Verify the Desks reappear

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>