Add Icon > Milestone

Purpose

Adds a new dedicated icon for Milestones. Milestones currently use clock, which will be retained. Once implemented we will need to identify all instances where clock is used for milestones and replace with milestone.

Screenshots

Checklist

For a new icon and/or illustration:

• Link to related design issue: #404 (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 #404 (closed)