Fix radii of MR thread nav buttons

What does this MR do and why?

Fix radii of MR thread nav buttons

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Check out this branch
2. Visit an MR with resolved threads
   1. You will see that the All threads resolved! nav has the same border radii as buttons
   2. Hover over the three-dot menu
   3. You will see that it has a flat left side and its right border radii match the parent
3. Visit an MR with unresolved threads
   1. You will see that the thread nav element has the same border radii as buttons
   2. You will see that the "Previous open thread" and "Next open thread" buttons do not have rounded corners
   3. You will see that the three dot menu has a flat left side and its right border radii match the parent

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.