Adds an option, `bind_new_user_emails_to_sydent`, which uses Sydent's [internal bind api](https://github.com/matrix-org/sydent#internal-bind-and-unbind-api) to automatically bind email addresses of users immediately after they register.

This is quite enterprise-specific, but could be generally useful to multiple organizations. This aims to solve the problem of requiring users to verify their email twice when using the functionality of an identity server in a corporate deployment - where both the homeserver and identity server are controlled. It does with while eliminating the need for the `account_threepid_delegates.email` option, which historically has been a very complicated option to reason about.