Skeletons of the new Notification and Session systems

Partly, just checking in to test that I understand how to put this on a
branch.
Beginnings of System Messages

Added a trivial UI for writing system messages, and pushed them all the way
through to the Notifications system. They don't *do* anything yet, but it's
a start.
Progress towards initial Notifications.

Wound up implementing UserCache, largely by accident but it's been on the
to-do list anyway.
Finally replaced the blocking wait for the User record with a proper
Async. For the first time ever, this code is now right.
Begin to display Notification Alert. DB CHANGE!

IMPORTANT: note that 8.sql will need to be applied to the system DB.

Displaying the bell icon. Not actually hooked up to the actual number of
new notifications yet, but we'll get there soon.
UI now gets the number of notifications from the back end.

Still mocked up, but from further back in the stack.
Pushed things all the way to the Persister, finally.
Made the message pathways a little more correct.
We are now actually evolving up the User Notifications table.

This involved many incredibly stupid bugs due to the duplicate code between
Space and User evolution. Really need to refactor this.
We now load the current Notifications when we start the UserSession.

Of course, we aren't *creating* any Notifications yet. But we're close.
Finished hooking up the pieces to send a System Message.

Worked the first time, too, which is a fine change of pace.
Bugfix: it appears that a MySQL SMALLINT is still a Java Int.
I can now see my received messages.
DB CHANGE: we now actually track which notes you've glanced at

The Notification icon in the menu bar now updates properly.

Changed the DB representation of this to make more sense the way things
fell out. (In 8.sql)
Added notifications for Comments.

Only goes to the Space Owner at this point, but it's progress.

Gave IdentityCache the ability to route messages to UserSessions.

Managed to screw up several times; added better error handling to
SpacePersistence to cope.
Tweak so that we don't wind up with a huge pile of messages marked unread.
Comment notifications now include a link to go to the comment.

This doesn't quite work yet, due to the timing of link resolution, but at
least the URL is hypothetically right.
Navigating to a specific comment now works.
Bugfix: UserValues that were set on the Space itself weren't showing up in the UI
You can now control whether you get Comment Notifications in this Space.

Via the Advanced page. It's primitive and hokey, and doesn't take effect
until the Space next reloads, but it's better than nothing.