Run deployment jobs only in the parent project

Deployment jobs systematically - and expectedly - fail in the forked
projects, leading to confusing CI errors for the end user. Avoid the
confusion.

.gitlab-ci.yml

@@ -69,7 +69,7 @@ deploy preview:
     - .deploy eos
   stage: deploy
   rules:
-    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_PROJECT_NAMESPACE == 'cmsgemonline/gem-ops' && $CI_PIPELINE_SOURCE == 'merge_request_event'
   environment:
     name: preview-$CI_MERGE_REQUEST_IID
     url: https://cmsgemonline.web.cern.ch/doc/preview-$CI_MERGE_REQUEST_IID
@@ -83,7 +83,7 @@ deploy preview:stop:
   extends: .deploy eos:stop
   stage: deploy
   rules:
-    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_PROJECT_NAMESPACE == 'cmsgemonline/gem-ops' && $CI_PIPELINE_SOURCE == 'merge_request_event'
       when: manual
       allow_failure: true
   environment: