Ability to make an Announcement at the Monitor level (vs Status Page)#719

Provide the functionality to create an announcement at the monitor level. This way if a monitor is used in multiple status pages, subscribers to the status pages that use the particular monitor would get the notifications.

This would eliminate the need to create/uptade announcements one by one for each status page that use the same monitor.

This would also eliminate the need for subscribers to register on multiple status pages that have monitors at different levels in a hierarchy (parent-child-grandchild…) as they could simply register for the status page that has the lowest node