Related branches: add widget styles

What does this MR do and why?

Changes the «Related branches» to use the widget markup. Also moves the component to be placed after «Related merge requests».

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open an issue
2. Create a new branch
   1. Click on the arrow on the «Create merge request» button
   2. Select «Create branch»
   3. Click «Create branch» button
3. Go back to the issue
4. Check the «Related branches» component

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.

Related to UX Paper Cuts 15.11 → Merge request UI (#394879 - closed)