Enable MergeableSelector in scss-lint

Supplement of gitlab-org/gitlab-ce!12810 (Enable MergeableSelector in scss-lint).

FYI: List of conflicts between CE and EE

There are 10 conflicts over 10 SCSS files according to the latest Job #38175001:

    - app/assets/stylesheets/framework/common.scss:292

    - app/assets/stylesheets/framework/header.scss:354

    - app/assets/stylesheets/framework/lists.scss:299

    - app/assets/stylesheets/framework/selects.scss:60

    - app/assets/stylesheets/pages/builds.scss:68

    - app/assets/stylesheets/pages/environments.scss:287

    - app/assets/stylesheets/pages/issuable.scss:127

    - app/assets/stylesheets/pages/merge_requests.scss:350

    - app/assets/stylesheets/pages/note_form.scss:111

    - app/assets/stylesheets/pages/notes.scss:467

    - app/assets/stylesheets/pages/pipelines.scss:906

Ref.

• https://gitlab.com/tnir/gitlab-ce/-/jobs/38175001
• https://gitlab.com/gitlab-org/gitlab-ce/issues/23485#note_34920745

/label ~"technical debt" frontend ~backstage ~"Community Contribution"