Refetch the token when we get a badtoken error

Intercept badtoken errors, refetch the edit token from the
action=tokens API, and retry the request again. If this fails too,
show the error to the user.

Right now this just shows the good old confirm() dialog if the token
refetch fails; we should probaby give the user a clearer error message
telling them to refresh the page or something.

Bug: 42984
Change-Id: Ib43d1938ffa24bc8d1dc76a300e16e486dabd928
Merge "Refetch the token when we get a badtoken error"