webhooks: implement internal webhook users

AUTH_INTERNAL requests previously could not register webhooks. This
commit adds the necessary changes to allow for this.

all: pass errors to panic, not strings

GraphQL's recovery middleware can't handle strings so it just logs a
very not useful <nil>

webhooks: expand auth configuration

The purpose of this change is to enable internal webhooks to be
configured in GQL webhook tables. A webhook subscription now includes
the auth method field which is appropriate, which is limited to either
OAUTH2 or INTERNAL. In the former case, the previous set of fields will
be valid, and in the latter case, the NodeID field will be valid. This
will allow us to register webhook subscriptions for internal use.

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