Add new icons > xs sizes for × and ✓

Purpose

Add xs variants of the × and ✓ icons. Both use the existing 12px grid, but just reduce optical size.

Screenshots

Note: To preview the icons correctly in the review app, use the Extra small (12px) setting.

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.

Closes #385 (closed)