mw.ViewPageTarget: Refetch token if session expired

* Rephrased visualeditor-savedialog-error-badtoken to emphasise
  that it is the old session that become invalid, not the one
  the user started browsing with since in a different window.
* If the session changed, the user will be asked whether they
  agree to save with this new session instead.
* We explictly update mw.config so that future save attempts
  in the same window compare against the correct environment.
  Without this there are two problems when saving and then
  making a second edit in the same window and saving that:
  - It will bring up the same question again (user A -> user B),
    which is annoying.
  - If the user logged back in again (new session, but for
    user A again) it would silently try with that new token
    without asking, thus saving as user A when the user still
    thinks it switched to user B. It switching back automatically
    is not obvious since we asked them from A->B, so we should
    also ask the other way around.
  This can be reproduced by opending ve-edit logged-in, then
  logging out in a new window, save, confirm anon, save,
  open edit again, log back in in a new window, save open edit
  in the old window, confirm new logged-in, save.

Bug: 50424
Change-Id: Id055eca1886f85aeaf615f645de29898afc0373c