SYWEB-152: Move up/down history fully to a directive.

Previously, there was some of it in a lovely generic directive, but the
core of it was hard coded id attributes in RoomController. It's now all
generic in a directive: the room history you get when you up/down arrow
is determined by the value of the attribute e.g. command-history="!foo:bar"
would present the history for !foo:bar. In practice, this is {{room_id}}
in the html.