Fix instantly sending RRs

Splits UserActivity into a tristate of 'active' (last < 1s), 'passive' (lasts a
couple of mins) and neither. Read receipts are sent when 'active', read markers
are sent while 'passive'.

Also fixed a document / window mix-up on the 'blur' handler.

Also adds a unit test for UserActivity because it's quite complex now
(and changes UserActivity to make it testable by accessing the singleton
via sharedInstance() rather than exporting it directly).

Fixes https://github.com/vector-im/riot-web/issues/9023

src/Lifecycle.js

@@ -439,7 +439,7 @@ async function startMatrixClient() {
     dis.dispatch({action: 'will_start_client'}, true);
 
     Notifier.start();
-    UserActivity.start();
+    UserActivity.sharedInstance().start();
     Presence.start();
     DMRoomMap.makeShared().start();
     ActiveWidgetStore.start();