Add support for multipart/mixed + better fallback for multipart/*

swh/web/inbound_email/utils.py

...b/tests/inbound_email/resources/multipart_alternative.eml

...bound_email/resources/multipart_alternative_html_only.eml

...bound_email/resources/multipart_alternative_text_only.eml

swh/web/tests/inbound_email/resources/multipart_mixed.eml

swh/web/tests/inbound_email/resources/multipart_mixed2.eml

...sts/inbound_email/resources/multipart_mixed_text_only.eml

swh/web/tests/inbound_email/test_utils.py