GitHub webhooks integration

Instructions to setup the bridge:

(i) On GitHub, in organization settings, add a webhook to this URL:
https://notifications.nasqueron.org/gate/GitHub/Nasqueron

(ii) Here in storage/apps/credentials.json, add a object in services
array with the following propeties:

{
  "gate": "GitHub",
  "door": "Nasqueron",
  "secret": "secret123"
}

(iii) Add relevant code in GitHubGateController::onPayload