Add release event trigger (#3226)

Supersedes #764 

Bitbucket does not support release webhooks.

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Co-authored-by: Patrick Schratz <patrick.schratz@gmail.com>

server/forge/gitlab/gitlab.go

@@ -625,6 +625,8 @@ func (g *GitLab) Hook(ctx context.Context, req *http.Request) (*model.Repo, *mod
 		return convertPushHook(event)
 	case *gitlab.TagEvent:
 		return convertTagHook(event)
+	case *gitlab.ReleaseEvent:
+		return convertReleaseHook(event)
 	default:
 		return nil, nil, &forge_types.ErrIgnoreEvent{Event: string(eventType)}
 	}