Check validity of parsed email-adresses

[pajowu]

Sometimes public bodies send email headers which do not conform strictly to rfc2822, which can lead to invalid email-addresses being returned by our email parser. We could filter the parsed adresses with some basic validity checks (e.g. Contains an @).

One example (adopted from a real email):

>>> from froide.helper import email_parsing
>>> import io
>>>
>>> data = b"To: 'John Doe [#123]' <john.doe@example.com>"
>>> parsed_email = email_parsing.parse_email(io.BytesIO(data))
>>> parsed_email.to
[('', "'John"), ('', ''), ('', '#123'), ('', ''), ("'", 'john.doe@example.com')]