Support the Unread Messages feature in RTIR

Add support for displaying a message on ticket display when
there are unread messages on a ticket. This is a standard
feature in core RT.

Adding this feature gives users a way to clear the New messages
setting shown on the incidents listing on the RTIR homepage
which was added in 726463f367.

Resolves issues 17450 and 30080
Make "Jump to unread" work

Without this and 4.0/updatestatus-displaypath, Jump to unread will link
to /Ticket/Display.html?id=7#txn-id=foo but during the redirect to
/RTIR/Display.html the anchor is lost and the page never moves.  You can
click the link all you want.

Mark as read and jump works because it passes the Anchor in an argument
which is respected in MaybeRedirectForResults.
Merge branch '3.0/display-mark-as-seen' into 3.0-trunk