With perfectScrollbar, we needed to call a function from JavaScript to
enable a scrollbar on a new element, but simplebar has a much simpler
default API one can do by using data-simplebar attributes in the HTML.
So we can delete all the scrollbar creation/deletion code.
Signed-off-by: Anders Kaseorg <anders@zulipchat.com>
The legacy "Updated Successfully" message shown after saving changes,
is removed, and replaced with our standard "Saving" spinner and
animation.
Fixes: #11177.