Fix timeline markers not working on Chrome (#13887)

* Periodically save timeline markers

This saves timeline markers immediately upon message arrival, but not more
than once every 5 minutes.

This does not change how the markers are saved on closing the window,
except that it avoids submitting them if there is no need for it.

* Use the Fetch API when possible instead of XHR on window unload

app/javascript/mastodon/reducers/markers.js

@@ -0,0 +1,25 @@
+import {
+  MARKERS_SUBMIT_SUCCESS,
+} from '../actions/notifications';
+
+const initialState = ImmutableMap({
+  home: '0',
+  notifications: '0',
+});
+
+import { Map as ImmutableMap } from 'immutable';
+
+export default function markers(state = initialState, action) {
+  switch(action.type) {
+  case MARKERS_SUBMIT_SUCCESS:
+    if (action.home) {
+      state = state.set('home', action.home);
+    }
+    if (action.notifications) {
+      state = state.set('notifications', action.notifications);
+    }
+    return state;
+  default:
+    return state;
+  }
+};