Indicate which announcements are new and allow a user to mark an announcement as read

Problem/Motivation

Before committing #3206643: Project messaging channel in core (as experimental) to core we removed the ability to mark an announcement as new and as read from the module because the way it worked was inconsistent with how a user would expect this to work.

Merely revisiting the announcements page would mark all the announcements as read and the javascript to mark an announcement as read only had a visual affect. There also was no ability to mark an announcement as unread. This issue will scope out how this feature should work starting afresh.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet