Create new release when merge requests are merged into the
`$STABLE_BRANCH` branch.

* .gitlab-ci.yml: execute a new stage `release` for the new
  `semantic-release` include.

* release.config.js: create `docs/CHANGELOG.md` for stable branch and
  publish new releases on Gitlab.
  Mangle commits to have links for users, issues and commits id.
  Build npm package and publish the tarball with the release.

* release-rules.js: create semantic version update rules based on
  `docs/CONTRIBUTING.md`.