Change the logic for rendering PM threads in PM section to
be in the same as that of topics view --
In default view, only recent 5 PM threads would be shown
and append the active conversation as the 6th one at last
if not present in those 5.
In PM section with unreads, a maximum of 8 conversations
would be shown and rest of them would be hidden behind
the 'more conversations' li-item, clicking on which takes
to the zoomedIn view of PM section where all the present
PM threads would be visible.
Co-authored-by: Tim Abbott <tabbott@zulip.com>