Adjust layout of mr widget checkboxes

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR resets some alignment issues in the 'ready to merge' MR widget caused by gitlab-ui fixes upstream (gitlab-ui!3364 (merged)).

I've also made the spacing between checkboxes match Pajamas.

ITK thread on slack.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

For reference, here is how the widget used to look before the upstream changes broke the layout here. Notice the change to the spacing between items.

Before before	This MR

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create an MR that you have the correct permissions to merge. For example, create a new file called text.txt
2. View the MR widgets.

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.