Show dropouts on session monitor

[mrpg]

This may require a new event.

[mrpg]

We could also rewrite the existing ATTENDANCE event. Its associated data type is tuple[PlayerIdentifier, Optional[int]], which could be augmented. ATTENDANCE is needed by jobs.subscribe_to_attendance, which in turn is used only in the admin area.

[mrpg]

For future reference, here is the current purpose of some internal facilities:

• u.INFO is synchronized with the database and stores each player's id, page_order, show_page in-memory. u.INFO has a convenient structure, as it nests players within their session.
• u.ONLINE is a purely in-memory store to record when a player is on a page, performs some action. Entry is deleted upon player becoming a dropout, or leaving the page.-
• e.ATTENDANCE is an event that is (supposed to be) set when u.ONLINE changes.

[mrpg]

u.INFO and u.ONLINE now have the same macro-structure.

[mrpg]

Just noting that u.INFO no longer exists.

[mrpg]

So I guess this issue is now about dropouts being shown on the (now) session main.