Verify email addresses

[jancborchardt]

For security and usability reasons, the email address needs to be verified to make sure it works and the user owns it.

In case the email was changed, an additional note should be sent to the old email address in case someone else change the email address.

[PVince81]

Implemented in #27277 thanks to @imujjwal96, closing

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.