[New CDot Nav] Migrate Left navigation menu styles to gitlab-ui

Migrate reusable (non-CDot specific) sidebar styles to gitlab-ui. Most styles were introduced in these MRs:

• https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/7538
• https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/7668 Main styles live in components/_sidebar.scss

We should also move to use utility mixins directly on elements: https://docs.gitlab.com/ee/development/fe_guide/style/scss.html#utility-mixins. Comment here: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/8593#note_1632995171