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.

server: add server reference to context

webhooks: initial prototype for GQL-native webhooks

Fix bug with internal authentication

Make authorization errors more specific

server: set max upload size to 1 GiB

Stash remote address in context

Sign outgoing emails

Add legacy webhooks worker implementation

Expand context for work queues

server: use SO_REUSEADDR for main listener

server: add /query/api-meta.json, docs

server: incorporate queue management

Simplify server module

Add email work queue

Import GQL server interfaces from gql.sr.ht