Implement Milestone icon

Summary

We're adding a dedicated Milestone icon, bringing Milestone in line with other common attributes that utilize a unique icon and avoiding collision with other use cases for clock.

This icon will need to be applied to all contexts currently using clock for milestone. As there are cases where clock is used not for Milestone, we will need to identify these cases, update them, and verify they display milestone afterward.

Steps

• Merge milestone.svg into gitlab-svgs: gitlab-svgs!1043 (merged)
• Identify milestone icon usage
• Replace milestone icons with milestone
• Close this issue

Current milestone icon usage

• Issue list
  • List items
  • Milestone filter option
• Issue board
  • Milestone filter option
• Issue dashboard (Your issues)
  • Issue dashboard list item
  • Milestone filter option
• MR list
  • Milestone filter option
• MR dashboard
  • MR dashboard list item
  • Milestone filter option
• Issue detail: Collapsed sidebar milestone field
  • Related MRs
• Service desk
  • Milestone filter option
  • List item
• Child item (legacy)
• Child/linked item (work item)
• Linked item (legacy)
• Roadmap
  • Count in header
  • Filter option
• Code review analytics filter option
• MR analytics filter option
• Productivity analytics filter option
• Epic
  • Issue in epic
• Your work sidebar Milestones button

Clock uses that are not milestones and should not be updated (not exhaustive)

• CI/CD catalog item release date
• MR list Deployed date filters
• Commits Committed date filters
• Admin system started date
• Calendar activities
• User local time (profile/popover)