Server: Fix requestor e-mail validation

* No longer silently accept pattern 'a@b@c', otherwise stay compatibile.
* Valid patterns are: 'user@fqdn', '<user@fqdn>', 'user <user@fqdn>',
  'user surname <user@fqdn>', no Unicode support, multiple e-mails
  separated by comma are allowed.
* Replace email.utils.parseaddr() with extended regular expression.
* Remove import email.utils as no other users exist.

Signed-off-by: Pavel Kácha <ph@cesnet.cz>