Implement Default Webhooks (#4299)

Partially implement #770.
Add "Default Webhooks" page in site admin UI.
Persist to the existing webhooks table, but store with RepoID=0 and OrgID=0.
Upon repo creation, copy the set of default webhooks into the new repo.

models/repo.go

@@ -1366,6 +1366,10 @@ func createRepository(e *xorm.Session, doer, u *User, repo *Repository) (err err
 		return fmt.Errorf("newRepoAction: %v", err)
 	}
 
+	if err = copyDefaultWebhooksToRepo(e, repo.ID); err != nil {
+		return fmt.Errorf("copyDefaultWebhooksToRepo: %v", err)
+	}
+
 	return nil
 }