Add space between columns in runner table

What does this MR do and why?

This change improves the appearance of the runner table by adding space between runner table columns. It also removed some fixes widths to allow the contents to flow more freely in different layouts.

Fixes:

• Spacing between columns on larger screens match 16px as recommended by Pajamas
• The action buttons on the left don't overflow the screen
• Spacing between fields and the outer margin on smaller screens, when the table stacks

Screenshots or screen recordings

Before

After

How to set up and validate locally

0. Ensure some runners registered to the instance
1. Visit the admin runners page e.g. http://gdk.test:3000/admin/runners
2. Try different viewport sizes

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 #345385 (closed)