This commit introduces the change of rendering private messages
section as collapsible, whose data-fetching logic came with zulip#21357.
We now have separated out `Private messages` from `top_left_corner`
section and shifted it below the `global_filters` in a different
separate section along with stream list with common scroll bar
in left-sidebar.
The new PM section will be opened by-default on loading the page
and will have a toggle-icon in its header, clicking on which makes the
section collapse/expand accordingly.
In default view, only recent 5 PM threads would be shown
and would append the active conversation as the 6th one at last
if not present in those 5, similar to how topics list work.
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 and rest of the sections of left-sidebar
will get collapsed.
Fixes#20870.
Co-authored-by: Aman Agrawal <amanagr@zulip.com>
jai2201