mw.ViewPageTarget: Implement veaction=edit history state

Browser history state:
* Listen for popstate, if veaction is different, act on it
  (by calling activate or deactivate).
* When switching to edit mode, push veaction=edit.
  When switching to view mode, push non-veaction.
  Except if the switch was made by the popstate listener in
  which case we don't want to push again.

Clean up:
* Use document.location.href instead of odd location.toString
  Don't pass it to mw.Uri from the original one since it
  takes the current url by default.

Bug: 43844
Change-Id: I5c345c1526852a84b04436955845bf1b56d94402
Merge "mw.ViewPageTarget: Implement veaction=edit history state"