Tell users whether their unread messages (RM) are before or after their current read point

We used to assume that RMs occurred chronologically before whatever the user is currently reading. This may not be true if they are on a permalink however, so we changed the "jump to unread messages" bar to not have an upwards pointing button as a workaround.

Surely the server does know whether the RM is above or below our current read point however, and we should actually label the "jump to unread" bar correctly. This could be done by the server providing an API to let clients compare the chronologicity of two event ids.

cc @lukebarnard1