Support "content_subtype" EmailMultiAlternatives when use dict in serialization

[gianbot]

According with Django documentation:
https://docs.djangoproject.com/en/3.2/topics/email/#sending-alternative-content-types

By default, the MIME type of the body parameter in an EmailMessage is "text/plain". It is good practice to leave this alone, because it guarantees that any recipient will be able to read the email, regardless of their mail client. However, if you are confident that your recipients can handle an alternative content type, you can use the content_subtype attribute on the EmailMessage class to change the main content type. The major type will always be "text", but you can change the subtype

With this pull request we add content_subtype in to_dict and from_dict utils function to handle different content type when using DJANGO_Q_EMAIL_USE_DICTS is True.