Persist pinned message open-ness between room switches

Addresses part of https://github.com/vector-im/riot-web/issues/5405

Signed-off-by: Travis Ralston <travpc@gmail.com>

src/settings/Settings.js

@@ -274,4 +274,8 @@ export const SETTINGS = {
         displayName: _td('Enable widget screenshots on supported widgets'),
         default: false,
     },
+    "PinnedEvents.isOpen": {
+        supportedLevels: ['room-device'],
+        default: false,
+    },
 };