When streaming API is disconnected, poll home/notifications (#2776)

* When streaming API is disconnected, poll home/notifications
Display slightly different empty home timeline message if user is following others
Cull notifications to 20 items when over 40 get added in real-time
Run manage:translations

* Optimize <HomeTimeline /> a little

app/javascript/mastodon/locales/defaultMessages.json

@@ -713,6 +713,10 @@
       {
         "defaultMessage": "the public timeline",
         "id": "empty_column.home.public_timeline"
+      },
+      {
+        "defaultMessage": "Your home feed is empty. If you have been inactive for a while, it will be regenerated for you soon.",
+        "id": "empty_column.home.inactivity"
       }
     ],
     "path": "app/javascript/mastodon/features/home_timeline/index.json"