As a User, I would like to have webhook integration to subscribe to GH events

[SonOfLope]

To enable more visibility over contributions made to our main projects, It would help to have an integration in trema to subscribe to PR's being open on projects.

Acceptance criteria :

• new webhook endpoint on our API to accept webhook triggers from github with specific payload ('/github/uuid')
• The existing slash command to create webhooks should be compatible with this workflow.
• Enables at least to receive notifications for GH PR's being opened for a specific repo.
• Notification can be posted even in forum channels