Allow mounting arbitrary columns (#3207)

* Allow mounting arbitrary columns

* Refactor column headers, allow pinning/unpinning and moving columns around

* Collapse animation

* Re-introduce scroll to top

* Save column settings properly, do not display pin options in
single-column view, do not display collapse icon if there is
nothing to collapse

* Fix one instance of public timeline being closed closing the stream
Fix back buttons inconsistently sending you back to / even if history exists

* Getting started displays links to columns that are not mounted

app/javascript/mastodon/scroll.js

@@ -0,0 +1,29 @@
+const easingOutQuint = (x, t, b, c, d) => c * ((t = t / d - 1) * t * t * t * t + 1) + b;
+
+const scrollTop = (node) => {
+  const startTime = Date.now();
+  const offset    = node.scrollTop;
+  const targetY   = -offset;
+  const duration  = 1000;
+  let interrupt   = false;
+
+  const step = () => {
+    const elapsed    = Date.now() - startTime;
+    const percentage = elapsed / duration;
+
+    if (percentage > 1 || interrupt) {
+      return;
+    }
+
+    node.scrollTop = easingOutQuint(0, elapsed, offset, targetY, duration);
+    requestAnimationFrame(step);
+  };
+
+  step();
+
+  return () => {
+    interrupt = true;
+  };
+};
+
+export default scrollTop;