[CI] Add check for draft PRs (#5800)

* Add check for draft PRs

* Add necessary gitlab-ci boilerplate

* fix github API url (oops!)

* Add github action to retrigger pipeline

* fix workflow trigger

* rename trigger job

Co-authored-by: Benjamin Kampmann <ben@parity.io>

substrate/.gitlab-ci.yml

@@ -89,7 +89,16 @@ variables:
     - /^pre-v[0-9]+\.[0-9]+-[0-9a-f]+$/
     - web
 
+#### stage:                       .pre
 
+skip-if-draft:
+  image:                           parity/tools:latest
+  <<:                              *kubernetes-build
+  stage:                          .pre
+  only:
+    - /^[0-9]+$/                  # Pull requests
+  script:
+     - ./.maintain/gitlab/skip_if_draft.sh
 
 #### stage:                        test