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/index.js

@@ -35,6 +35,7 @@ import identity_proofs from './identity_proofs';
 import trends from './trends';
 import missed_updates from './missed_updates';
 import announcements from './announcements';
+import markers from './markers';
 
 const reducers = {
   announcements,
@@ -73,6 +74,7 @@ const reducers = {
   polls,
   trends,
   missed_updates,
+  markers,
 };
 
 export default combineReducers(reducers);