Simplify oauth_authorized with redirect

[cancan101]

repost from https://github.com/singingwolfboy/flask-dance/issues/203#issuecomment-486333141 (CC @daenney @singingwolfboy) Right now the expectation is that writers of oauth_authorized callbacks handle saving the token if they want a redirect. This leads to more complicated logic to handle redirects (see linked comment), reposted here: why push that responsibility on the user? Seems like there is a non trivial amount of logic in setting the token (ie handling errors) that I would guess most consumers would still want to use.

I think in the base case of just wanting to customize redirects (ie to send the user back to the where they originally came from) where the logic is:

    next_url = flask.session["next_url"]
    return flask.redirect(next_url)

having to also set the token seems unnatural.

I suggest improving the DX here to allow returning the a response and still re-using the existing set token logic.