Put always-on-screen widgets in top left

always-on-screen widgets now appear in the top-left where the
call preview normally is if you're not in the room that they're in.

Fixes https://github.com/vector-im/riot-web/issues/7007
Based off https://github.com/matrix-org/matrix-react-sdk/pull/2053

package.json

@@ -87,6 +87,7 @@
     "react-beautiful-dnd": "^4.0.1",
     "react-dom": "^15.6.0",
     "react-gemini-scrollbar": "matrix-org/react-gemini-scrollbar#5e97aef",
+    "resize-observer-polyfill": "^1.5.0",
     "sanitize-html": "^1.14.1",
     "text-encoding-utf-8": "^1.0.1",
     "url": "^0.11.0",