Bug 31810: Define 'Place hold' button as primary

Bug 31810: Define 'Place hold' button as primary

When placing a hold in staff, the 'Place hold' button should be yellow for the most important action on this form.

Patch changes this button from btn-default to btn-primary.

Test plan :
Play with placing hold, one and several, with cases :
- normal
- override required
- none available

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>