[change] Allow passing extra arguments to django send_email function

[nemesifier]

The openwisp_utils.admin_theme.send_email function does not allow passing attachments or other arguments to EmailMultiAlternatives (which is a subclass of EmailMessage).

Let's make it more flexible by allowing to pass **kwargs so that we can allow any feature supported by Django's EmailMessage class.

Let's document this possibility by mentioning that any keyword argument supported by Django's EmailMessage class can be passed.