Enable no-duplicate-mixins rule
gitlab-org/gitlab-ui#431 (closed) requires that we enable the no-duplicate-mixins
rule that we contributed upstream in https://github.com/kristerkari/stylelint-scss/pull/371.
This MR upgrades stylelint-scss
from v3.8.0
to v3.12.0
and enables the new rule.
Changelog: https://github.com/kristerkari/stylelint-scss/blob/master/CHANGELOG.md#3120
Edited by Paul Gascou-Vaillancourt