Update avatar border to be opaque for better stacking

What does this MR do?

Update avatar border to be opaque for better stacking

Before	After

Are there points in the code the reviewer needs to double check?

Issue with multiple assignees (try with 2 and 5 assignees to see .avatar-counter)

• Boards
• Issue sidebar when collapsed

Why was this MR needed?

UI polish to keep stacked avatars from blending into each other.

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/32561