Use icon button for subscribe to rss feed in MR list

What does this MR do and why?

• Use btn-icon for subscribe to RSS feed/calendar buttons
• Adds tooltip on hover
• Matches the issues list and reduces clutter

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Look at MR list

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.