Replace Font Awesome cog icon with GitLab settings icon

Purpose

Replacing instances of fa-cog Font Awesome icons with GitLab settings icon.

Epic: &2331 (closed)

Reference:

• See https://leipert-projects.gitlab.io/is-gitlab-pretty-yet/icons/ for all Font Awesome icon instances
• See http://gitlab-org.gitlab.io/gitlab-svgs/ for all GitLab icons