Ignore messages with no content type

They are status updates and other junk
Dedup to

Google Voice puts the number in both to and cc for some reason...
Enable RTS threading
Fix some CI failures
Fix for new purebred-email
Convert MM4 mails to Bandwidth MMS JSON and send to a webhook
Expose some of the email helpers for use elsewhere
Update to latest purebred-email
Ignore dist-newstyle for lint
Add to and cc headers as XEP-0033
Extract and store email attachments and related media parts
jabber:iq:gateway working against my local Gajim
vcard-temp for the gateway

Includes needed license information.
Add install instructions to README
We need newer purebred-email
Get artifacts from CI
Fix namespaces on attributes

It turns out that attributes should normally *not* have explicit namespaces.
This is because the XML spec says that an attribute with no namespace is
implicity namespaced by the element it is on (to a pseudo namespace partition).

I think this was only ever working because the XML text generator in the XMPP
library is primitive and wasn't adding prefixes.
Use localpart as fallback display name

Using bare JID is nice, but is detected by many SPAM filters as an attempt to
spoof an email address.