mw.ViewPageTarget: Tag our state changes and reject foreign states

Store a bit of data with the states we push or replace in the
history so that when the user navigates back to them, we know
for sure this is a state we pushed in the history.

This allows us to filter out popstate events triggered by the
user browsing to states create by other software, as well as
states triggered by the browser that have no state data at all
(Chrome is known to, in contrast with other browsers, trigger a
blank popstate event on load, which we were mistaking for a user
event where the user navigates back to veaction=edit).

Bug: 57901
Change-Id: I142777d0d2ae96d3afee224782f0d2d1522da1eb