Fix subscribe to label dropdown

What does this MR do and why?

• Use caret for dropdown
• Align dropdown to right

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Go to project labels page (eg http://localhost:3000/gitlab-org/gitlab-shell/-/labels)
• Promote a project label to group level (if needed)
• Look at Subscribe dropdown at far right

#398228 (closed)