email address validation

[arnounesco]

the email validation on the download page seems to be broken

• valid email addresses (e.g. a.lambert+obis@unesco.org ) are rejected
• invalid email addresses (e.g. a.lambert@unesco.asdfasdfasdfasdfasdfasdfasdfasfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasfasdfasfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdf) are accepted, resulting in returning undeliverable mails

see also #138