Fix the CI preview feature for the non-developer members

It appears that the "Run Pipeline" button in the "Pipeline" tab of the
merge requests is enabled only when a pipeline exists in the forked
project. There is thus no possibility to deploy the MR to a preview
envronment for non-developer members.

This commit ensures that a merge request pipeline exists in the forked
project.

.gitlab-ci.yml

@@ -70,6 +70,9 @@ deploy preview:
   stage: deploy
   rules:
     - if: $CI_PROJECT_NAMESPACE == 'cmsgemonline/gem-ops' && $CI_PIPELINE_SOURCE == 'merge_request_event'
+      when: always
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+      when: manual
   environment:
     name: preview-$CI_MERGE_REQUEST_IID
     url: https://cmsgemonline.web.cern.ch/doc/preview-$CI_MERGE_REQUEST_IID
@@ -83,7 +86,7 @@ deploy preview:stop:
   extends: .deploy eos:stop
   stage: deploy
   rules:
-    - if: $CI_PROJECT_NAMESPACE == 'cmsgemonline/gem-ops' && $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
       when: manual
       allow_failure: true
   environment: