GitLab color definition missing

[tlindsay42]

Description

The GitLab social icon is defaulting to black because there is no specification.

Environment

• Minimal Mistakes version: 4.11.2
• Ruby gem or remote theme version: Remote Theme
• Jekyll version: 3.7.3
• Git repository URL: https://github.com/troylindsay/troylindsay.github.io
• Operating system: Windows 10
• GitHub Pages hosted (if yes provide URL to site): https://www.troylindsay.io

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Expected behavior

The GitLab icon in the author profile sidebar is set to one of the official brand colors.

Steps to reproduce the behavior

Set the GitLab username in _config.yml

author:
  gitlab: 'tlindsay42'

While served, the GitLab icon that appears in the author profile is black because there is no specification for it in _variables.scss or _utilities.scss.

Pull request following momentarily.