ui: storage: show no-keeps hint exactly when needed

ui: storage: show no-keeps hint exactly when needed

extracting the logic from the previous checkbox listener into a function, which
is also called on field changes and once in afterrender. Calling it initially
makes sure the hint is also displayed at the beginning when editing a storage
with no retention options configured, and the initial disabling of the input
fields for the isCreate case now also happens through that call.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>