Bug 30108: (follow-up) Add required indication, differentiate alert

Bug 30108: (follow-up) Add required indication, differentiate alert

As asked on comment4:
[1] Add a required div as a visible hint when applicable.
[2] Differentiate alert for hold start and end date.

Test plan:
See former patch.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>