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

Bugfix, mail config placed in wrong setting.

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

Add support for mail setups that don't support TLS or Auth

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

email: remove leftover debug code

Add email work queue