account for MIMEBase attachments

[joshuadavidthomas]

It's possible to attach a MIMEBase object directly according to the Django docs:

• attach() creates a new file attachment and adds it to the message. There are two ways to call attach():

  • You can pass it a single argument that is a MIMEBase instance. This will be inserted directly into the resulting message.

This PR accounts for that possibility when serializing an email to JSON when saving to the database.

It also removes unreachable code in the email property getter, as when the setter saves the serialized email in the database, it always saves the attachment as a dict.