Let users change their email

[brndd]

Changing email should be a basic feature of any web service. At Tampere Hacklab, the board gets a couple of requests per year to change user emails, and currently we can't do this without possibly breaking things like Nextcloud or Matrix, because changing the email changes the user's identity in SSO.

The simplest way to solve this would be to keep the current email as a login and nothing else, and add a separate user-changeable email address that's used for actual email communications. This could be mildly confusing, but much better than not being able to change email at all.