[enh] Add github-sync stage

.gitlab-ci.yml

+stages:
+    - github-sync
+
+push_to_github:
+    stage: github-sync
+    variables:
+        GIT_STRATEGY: none
+    tags:
+        - github
+    script:
+        - rm -rf ./*
+        - rm -rf .git
+        - git clone --mirror $CI_REPOSITORY_URL .
+        - git remote add github $GITHUB_URL_AND_KEY
+        - git config --global user.email "contact@duniter.org"
+        - git config --global user.name "Duniter"
+        # Job would fail if we don't remove refs about pull requests
+        - bash -c "cat packed-refs | grep -v 'refs/pull' > packed-refs-new; echo 'Removed pull refs.'"
+        - mv packed-refs-new packed-refs
+        - bash -c "git push --force --mirror github 2>&1 | grep -v duniter-gitlab; echo $?"