RTC: [Interface] Notifications for session activity

[maxschmeling]

The following actions should trigger toast notifications:

• A new collaborator joins the session
• A collaborator leaves the session
• The post is saved
• The post is published

[smithjw1]

We've also seen a desire to default some of these to "on" until users turn them off. I don't think we need UI, but perhaps a filter to set the default for an application.