ci(commitlint): enforce commit message format

Prepare the use of `semantic-release` by forcing commit message
format.

* .gitlab-ci.yml: `$STABLE_BRANCH` is `master`.
  Execute `commitlint` before resource heavy jobs at `initial-checks`
  stage.

* .eslintignore: exclude `commitlint` configuration file.

* .prettierignore: exclude `commitlint` configuration file.