Support storing sensitive notification settings securely/encrypted.
Move slack notifier url and api token to secure settings.
Migrating slack notifier to store token and url encrypted is currently 
a manual process by saving an existing slack alert notification channel.
saving an existing slack alert notification channel will reset the stored 
non-secure url and token.

Closes #25113
Ref #25967

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>