/help/: Re-add perfectScrollbar for sidebar and markdown.
/help/: Re-add perfectScrollbar for sidebar and markdown.

This adds back the perfectScrollbar for the sidebar and markdown
sections because we already lost CTRL-F functionality, so we may
as well bring back the pretty, non-obtrusive scrollbars.

This updates the scrollbar after a successful `slideToggle` of
one of the sidebar sections.

Fixes: #6999.

hackerkid/zulip,jackrzhang/zulip,shubhamdhama/zulip,brainwane/zulip,rht/zulip,jackrzhang/zulip,rht/zulip,showell/zulip,mahim97/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,mahim97/zulip,Galexrt/zulip,rishig/zulip,rht/zulip,zulip/zulip,punchagan/zulip,kou/zulip,rht/zulip,hackerkid/zulip,brainwane/zulip,kou/zulip,eeshangarg/zulip,timabbott/zulip,mahim97/zulip,jackrzhang/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,jackrzhang/zulip,timabbott/zulip,zulip/zulip,zulip/zulip,shubhamdhama/zulip,dhcrzf/zulip,dhcrzf/zulip,punchagan/zulip,andersk/zulip,mahim97/zulip,shubhamdhama/zulip,hackerkid/zulip,shubhamdhama/zulip,rishig/zulip,synicalsyntax/zulip,brockwhittaker/zulip,timabbott/zulip,andersk/zulip,shubhamdhama/zulip,tommyip/zulip,punchagan/zulip,eeshangarg/zulip,brainwane/zulip,Galexrt/zulip,kou/zulip,brockwhittaker/zulip,brockwhittaker/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,tommyip/zulip,synicalsyntax/zulip,synicalsyntax/zulip,rishig/zulip,timabbott/zulip,mahim97/zulip,rishig/zulip,synicalsyntax/zulip,tommyip/zulip,tommyip/zulip,dhcrzf/zulip,brockwhittaker/zulip,Galexrt/zulip,showell/zulip,andersk/zulip,Galexrt/zulip,Galexrt/zulip,showell/zulip,Galexrt/zulip,eeshangarg/zulip,synicalsyntax/zulip,Galexrt/zulip,brainwane/zulip,kou/zulip,hackerkid/zulip,kou/zulip,showell/zulip,tommyip/zulip,shubhamdhama/zulip,brockwhittaker/zulip,zulip/zulip,brainwane/zulip,andersk/zulip,synicalsyntax/zulip,shubhamdhama/zulip,brainwane/zulip,tommyip/zulip,rishig/zulip,timabbott/zulip,hackerkid/zulip,andersk/zulip,rishig/zulip,jackrzhang/zulip,mahim97/zulip,zulip/zulip,brockwhittaker/zulip,showell/zulip,timabbott/zulip,punchagan/zulip,punchagan/zulip,punchagan/zulip,andersk/zulip,synicalsyntax/zulip,dhcrzf/zulip,showell/zulip,brainwane/zulip,zulip/zulip,dhcrzf/zulip,eeshangarg/zulip,dhcrzf/zulip,jackrzhang/zulip,jackrzhang/zulip,tommyip/zulip,kou/zulip,dhcrzf/zulip,rht/zulip,rishig/zulip,timabbott/zulip