Mark unread messages in the room as read when all messages are visible

This is an alternative to network/neochat!467. When discussing in the matrix channel this option seemed more popular so I implemented it.

Mark the unread messages in the room as read when all messages are visible to the user after a short timer. This happens on entry and when new messages come in as long as Neochat is active. If neochat isn't active, the room hasn't loaded or the read marker hasn't loaded the timer is for this is reset until all 3 conditions are false.