Notification for low disk space on Pulp
The notifications drawer should show a warning when we are running out of disk in any Pulp proxy.
Fixes #19302 - Notification for Pulp low disk space
If some proxy is running out of disk space, this notification will warn
the user about it. This will check all proxies with the Pulp feature for
storage on a cron and create notifications according to that. IF the
notification already exists, then it will not create a new one.