Anonymization of password recovery emails on homeservers

This issue has been migrated from [#1880](https://github.com/matrix-org/synapse/issues/1880).

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Some people (me, for instance), upon registration to a Matrix home server, shy away from submitting their email address for PW recovery, for a variety of reasons.

I have a suggestion that would allow users to "postpone" the decision on trusting the home server with their email address until they actually need it:

==> Upon registration with a home server, the user could submit a self-generated TBD hash of the email address (s)he wishes to use for eventual password recovery. Only when the user actually forgets his/her password, would (s)he need to submit the email in clear-text. The server uses the stored hash to verify the email address and sends the password recovery link.

1) For further privacy friendliness, a trustworthy home server would never store the email in clear text
2) For added security, the home server could store a salted hash of the user submitted hash. That way, even if the home server (and all it's databases) are compromised, no one can link users to email addresses by the use of hash tables for known email databases