server/email: simplify and cleanup

- Only print the error once
- Remove unnecessary var
- Use debug.Stack instead of hand-rolled logic
- Ignore json.Marshal errors in a simpler way

go fmt

server/email: handle errors out of GQL context

server/email: include variables

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.

Sign outgoing emails

server: incorporate queue management

Add email work queue

Import GQL server interfaces from gql.sr.ht