email: remove content headers of passed-in message

The email has already been parsed according to the headers, but they are
still present. However, signing or encrypting the email will change the
content format, so remove any such headers before continuing.

email: handle error from Enqueue() in EnqueueStd()

email/worker: add more context to errors

For instance, failure to open the private PGP key would just fail
with an unhelpful "panic: open : no such file or directory" error.

email.EnqueueStd: Don't overwrite headers

Sometimes we need to specify the Message-Id, From, and Reply-To headers
(e.g. for todo.sr.ht ticket notifications). Don't overwrite these
headers if they are present.

email: handle failure to encrypt notifications

Currently, failure to encrypt the notification email - e.g. due to an
expired key - will make the entire request fail. Instead, fall back to
sending an unencrypted email and let the request succeed (unless that
fails also, which is unlikely).

go fmt

Replace x/crypto package by ProtonMail/go-crypto to support ed25519 OpenPGP keys

go fmt

Remove %e formatting verbs

%e is not valid for formatting error values:

> %e	scientific notation, e.g. -1.234456e+78

Instead, use %w when using fmt.Errorf (for error wrapping) and
%v when using log.Printf (%w is not valid in that context).

Configure server to manage mail queue

This also sets up the recovery function for webhooks, so we get emailed
when we panic during a webhook query.

Encrypt outgoing emails

Sign outgoing emails

Make worker logs less loud

Expand context for work queues

Add email work queue