As Admin, I can create a System Notification

Summary: That is, I can send a message to all Querki users. This will be hugely useful to me as we go forward, so we should probably start with it.

This probably should not be added to each user's Notification table immediately. Instead, it should probably go into their Notification table the next time they log in, possibly as a simple link to a master copy.

OTOH, adding a system-wide Notifications table is its own particular brand of Special. It may be easier for now to copy it into each User's Notifications table (creating that table if it doesn't exist), and treat the common system table as a special case. That would force us to deal with "add a Notification to an unlogged-in user", which we are going to need to do anyway soon.