Add face-neutral and face-unhappy icons

Purpose

• Add two new emoji-like icons to support #312 (closed).
• These new icons have been prefixed with face- for naming to begin to categorize them. The current face icon names were preexisting and there are no plans to update them at this time since the process is rather laborious (duplicate icons with new names would have to be added, then each instance updated before finally deprecating the icons with the old names).

Screenshots

https://gitlab.com/gitlab-org/gitlab-svgs/uploads/b81cdab7befe108e8ad0a970be732dd0/image.png

Checklist

For a new icon and/or illustration:

• Link to related design issue:
• If this adds an icon ensure that it follows the new icon checklists.
• If this adds an illustration ensure that it follows the new illustration checklists.

For third-party trademarks:

• If this adds or updates an icon or illustration that includes a third-party trademark, ensure that it follows the process for adding third-party trademarks to GitLab.

Relates to #312 (closed)