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/components/column.js

@@ -0,0 +1,45 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+import scrollTop from '../scroll';
+
+class Column extends React.PureComponent {
+
+  static propTypes = {
+    children: PropTypes.node,
+  };
+
+  scrollTop () {
+    const scrollable = this.node.querySelector('.scrollable');
+
+    if (!scrollable) {
+      return;
+    }
+
+    this._interruptScrollAnimation = scrollTop(scrollable);
+  }
+
+  handleWheel = () => {
+    if (typeof this._interruptScrollAnimation !== 'function') {
+      return;
+    }
+
+    this._interruptScrollAnimation();
+  }
+
+  setRef = c => {
+    this.node = c;
+  }
+
+  render () {
+    const { children } = this.props;
+
+    return (
+      <div role='region' className='column' ref={this.setRef} onWheel={this.handleWheel}>
+        {children}
+      </div>
+    );
+  }
+
+}
+
+export default Column;