Add missing repo checks for incoming webhooks
Created by: eseliger
The first commit contains the pieces that I think are required in order to fix this bug.
The second commit is my initial attempt to implement the actual fix, I was stuck though when I realized that in the repo table the externalservice is not referenced by it's id :( I wanted to use the tuple [externalRepoID, externalServiceID, webhookKind] in order to fetch the correct repository to use when attaching the event.
Imo the best fix for this would be to have unique URLs for the webhooks that are directly associated to an external service and hence the repo fetching is simple. But my attempt is trying to work around that by making an assumption that a secret is always used just once across all external services