deps: Upgrade and move perfect-scrollbar from static/third to npm.

- Remove `perfect-scrollbar` from `static/third` and fetch it from npm.
- Upgrade `perfect-scrollbar` to 0.7.1.
- Bump up the `PROVISION_VERSION` to 5.6.

Changed `wheelSpeed` in "static/js/scroll_bar.js" to 0.5, because when it
20, the scrollbar scrolls very fast.

Changed 'wheelSpeed' in "static/js/emoji_picker.js" from 25 to 0.68
(based on tabbott's testing of scrolling through the emoji list).

Part of #1709.

static/js/scroll_bar.js

@@ -2,7 +2,7 @@ $(function () {
     $("#stream-filters-container").perfectScrollbar({
         suppressScrollX: true,
         useKeyboard: false,
-        wheelSpeed: 20,
+        wheelSpeed: 0.5,
     });
 });