MDL-9070 forum: Show timed post display period
This adds the following changes when viewing a list of discussions and
forum_enabletimedposts is turned on.
* Discussions a student wouldnt see right now are dimmed (timestart in
future or timeend in the past).
* A icon appears next to the discussion subject, the tooltip for this
icon displays the display start and/or end time. This only appears for
the user that made the discussion or has permission to
viewhiddentimedposts.
These two changes achieve a couple of important things, namely, teachers
will not mistakenly think their students can see a discussion when
viewing the list if they've later forgot that they set a time limit on
it (or indeed if someone else put it on their discussion without them
realising). Additionally it makes it easy to see from the list when
different discussions are set to be released without having to go in and
edit them to find out.
Change-Id: Ia1818b33c00adae0dad72df8e170a078fb66cd76
