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/actions/columns.js

@@ -0,0 +1,40 @@
+import { saveSettings } from './settings';
+
+export const COLUMN_ADD    = 'COLUMN_ADD';
+export const COLUMN_REMOVE = 'COLUMN_REMOVE';
+export const COLUMN_MOVE   = 'COLUMN_MOVE';
+
+export function addColumn(id, params) {
+  return dispatch => {
+    dispatch({
+      type: COLUMN_ADD,
+      id,
+      params,
+    });
+
+    dispatch(saveSettings());
+  };
+};
+
+export function removeColumn(uuid) {
+  return dispatch => {
+    dispatch({
+      type: COLUMN_REMOVE,
+      uuid,
+    });
+
+    dispatch(saveSettings());
+  };
+};
+
+export function moveColumn(uuid, direction) {
+  return dispatch => {
+    dispatch({
+      type: COLUMN_MOVE,
+      uuid,
+      direction,
+    });
+
+    dispatch(saveSettings());
+  };
+};