Resolve "New product icon > reply"

Purpose

Add a new icon that can be used for a comment reply instead of the current comment icon. This supports gitlab-design#2240 (moved).

Checklist

For a new icon and/or illustration:

• Link to related design issue: #375 (closed)
• 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.

Closes #375 (closed)