This transforms seenTime concept to a global property for all wikis
and sources, and updates the global seen time on opening the popup.
Bug: T134855
Change-Id: I67bcc4b346237317c7a9204dd43cd0e9ee02792f
Instead of 'cheating' and externally aggregating the itemUpdate event
we should just let the relevant (non-xwiki) models aggregate this
centrally and properly, and include it as an offered event.
Change-Id: Ibe528fe971e1be8309a97275b1a1be8979306ff5