prevent null bytes in email field

How it was found

Hi, Zawad here.

Description I recently visited gratipay.com and logged in. I found that invalid characters ( eg. null bytes ) were accepted in email field, which is obviously not an expected behavior.

Steps to Reproduce

1. Go to https://gratipay.com/~username/emails/
2. Enter [email protected]\0 or [email protected]%00 or [email protected]$
3. Now Inspect Element the field and change field type to text from email
4. Click on Add email address. You'll see an error message Looks like you've found a bug! Sorry for the inconvenience, we'll get it fixed ASAP! Now reload the page and you'll see the email listed.

It means server side validation of email is not okay. Validation should be improved.

For your reference, #3227 and #3991 are same kind of bug which were resolved.

Zawad