Use gitlab generic webhooks instead of drone-ci-service (#620)

Benefits:
- the webhook delivery history of the drone-ci-service is broken (no way to check if a webhook was successfully delivered by Gitlab)
- drone-ci-service has limited events support (for example no comment or branch deleted event)
- independent from drone integration in general

shared/token/token.go

@@ -52,10 +52,9 @@ func parse(raw string, fn SecretFunc) (*Token, error) {
 }
 
 func ParseRequest(r *http.Request, fn SecretFunc) (*Token, error) {
-	token := r.Header.Get("Authorization")
-
 	// first we attempt to get the token from the
 	// authorization header.
+	token := r.Header.Get("Authorization")
 	if len(token) != 0 {
 		log.Trace().Msgf("token.ParseRequest: found token in header: %s", token)
 		bearer := token
@@ -65,6 +64,11 @@ func ParseRequest(r *http.Request, fn SecretFunc) (*Token, error) {
 		return parse(bearer, fn)
 	}
 
+	token = r.Header.Get("X-Gitlab-Token")
+	if len(token) != 0 {
+		return parse(token, fn)
+	}
+
 	// then we attempt to get the token from the
 	// access_token url query parameter
 	token = r.FormValue("access_token")