This fixes the following flow being broken:
* You start on
http://zulipdev.com:9991/accounts/go/?next=/upgrade%23sponsorship
(which we link to from e.g. /plans/)
* The form on that page has
`action=/accounts/go/?next=%2Fupgrade%23sponsorship`, i.e. it has correctly
URL-encoded the `next `value.
* You enter a realm name and hit submit.
* That redirects you to
`http://realm-26.zulipdev.com:9991/upgrade#sponsorship`, would is
correct if you are already logged in.
* However, if you're not logged in, `/upgrade` will serve a redirect
to the login page, landing you at
`http://realm-26.zulipdev.com:9991/devlogin/?next=/upgrade/#sponsorship`.
This page works. But in production, it would instead be:
`http://realm-26.zulipdev.com:9991/login/?next=/upgrade/#sponsorship`.
On this page, password login works but social login does not.
Note that the `next=` value is no longer URL-encoded, and thus is
parsed by the browser as `?next=/upgrade` + a hash of
`#sponsorship`.
* If you now login with Google auth, you find yourself on
`http://realm-26.zulipdev.com:9991/#sponsorship` (no /upgrade).
The root cause was that we have a bit of JavaScript in signup.js and
dev-login.js that is intended to handle this; but it was broken for
the social login case for unknown legacy reasons.
This bug appears to date from the original
b62bdde303 which introduced `next`
support for social backends in the first place.
ab2ef76baa