Enforce complexity limit on webhook payload

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.

webhooks: add query validation function

The new function tests a query against a GraphQL schema and returns any
validation errors if they are found. This is useful for determining if a
GraphQL query will pass validation when creating a new webhook.

webhooks: refactor exec code into separate func

webhooks: initial prototype for GQL-native webhooks