Validate Edinburgh emails

Additional check for valid Edinburgh student email addresses. If your
email begins s + 7 numbers + @, then the remainder of your email
address should be ‘sms.ed.ac.uk’.
Set Content-Type header in outbound emails

since some email clients are apparently fussy about this, even if
you’re just sending plain text. Also set character set to UTF-8 in case
of special characters in the participant’s name.