Move activating/deactivating to ViewPageTarget

That's where they belong IMO, since ViewPageTarget is the one that
has .activate() and .deactivate(), and mw.Target doesn't retain any
state (apart from some caching things).

Change-Id: Ia6cf5bac9054163d54ab492d691d8ce9d6a3bb90
Merge "Move activating/deactivating to ViewPageTarget"