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- Fixed the redirect URL generation of the Facebook and Google client.
- Changed the name of the post login redirect cookie from ``next`` to
``authlib-next`` to hopefully prevent clashes.
- Authentication providers may also return ``None`` as email address;
handle this case gracefully by showing an error message instead of
crashing.
- Pass full URLs, not only paths to the OAuth2 libraries because
otherwise, secure redirect URLs aren't recognized as such.