prevent null bytes in email field

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