In commit 6f9e97921, the last use of `pm_list_data.is_all_privates`
was removed when we restructured how private messages are shown
in the left sidebar. Removes the function since it is now no longer
relevant.
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>
