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/features/ui/containers/columns_area_container.js

@@ -0,0 +1,8 @@
+import { connect } from 'react-redux';
+import ColumnsArea from '../components/columns_area';
+
+const mapStateToProps = state => ({
+  columns: state.getIn(['settings', 'columns']),
+});
+
+export default connect(mapStateToProps)(ColumnsArea);