More problems with email validation

[thet]

• These incorrect mails would match OK: test@aha.ok hello, test@aha.ok, test@okay.aha okay@aha.test
  A change of the regex to ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$ would fix that.

• TLDs longer than 4 characters need to be supported. Due to the current non-strict validation regex, it would match OK. But with the change above we need to adapt the regex to support valid mails like: me@you.online with a TLD longer than 4 characters.

The other regex problems are still unsovled:

• Email regex does not allow apostrophes #6 (with an suggestion to use Plone's email regex)
• The EmailField does not allow umlaut-domains (or domains with other characters) #9
• Email regex does not allow ampersand (&) in the username #15

Here is another mail regex suggestion which seems to be quite complete:

• https://stackoverflow.com/a/201378/1337474
• https://emailregex.com/ (same as above)

[gyst]

Fixed in #40