* Drop support for delegating email validation
Delegating email validation to an IS is insecure (since it allows the owner of
the IS to do a password reset on your HS), and has long been deprecated. It
will now cause a config error at startup.
* Update unit test which checks for email verification
Give it an `email` config instead of a threepid delegate
* Remove unused method `requestEmailToken`
* Simplify config handling for email verification
Rather than an enum and a boolean, all we need here is a single bool, which
says whether we are or are not doing email verification.
* update docs
* changelog
* upgrade.md: fix typo
* update version number
this will be in 1.64, not 1.63
* update version number
this one too
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