No need to manage this seperately, it's all related and only brings in a single dep.

This helps servers hosting many actors, and it helps us when delivering to lots of people on the same server; we only need to make 1 request per server.

MailMate, for example, using quoted-printable encoding when lines are about 80 chars long. Blegh!

RFC822 makes Spamassassin unhappy and I learned isn't actually compliant with revisions to the message spec!

When we receive activity that has a collection, we see if any of the other actor recipients have the collection set as their followers. Then we can encode it with plus addressing. If we can't do that we drop it. Not ideal, but it covers most use cases. Next up we can encode arbitrary collections like "some/path/to/thing@example.com".

The receiving server can forward the message for us. For us to resolve the entire collection is a lot of work, especially since paginated followers collections only contain like 12 entries per page! Lots of people have over 300 followers.

A nil key gets handled by rsa.Sign* but we won't catch an empty pubkey url until some server finds an incorrect HTTP signature.

While here make apget print mail format by default, consistent with rest of system.

This lets it send activity which cannot be converted to mail such as Follow

This is actually valid according to the spec. This makes embedded Activities like Mentions neater, too.

Yet! Decoding shouldn't be too hard. But for now it's a TODO. At least error out so we don't send out crazy messages.

Clearer: set a body to read from, then change it when necessary. Fewer instructions! Also, mail clients don't really know what to do with text/markdown, so just use text/plain; that's what Gruber would want. Adds basic test case too