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
