Fix apparent spacing of issuable sidebar content

What does this MR do and why?

Issuable sidebar sections that contain block-level elements like avatars or labels appear to be closer to their section headers than text content. This MR adds some extra spacing to make the apparent spacing for these elements match the apparent spacing for text.

This MR also adjusts the layout of the Participants section header.

MR acceptance checklist

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

Screenshots or screen recordings

MR sidebar

Before	After

Issue sidebar

Before	After

How to set up and validate locally

1. Check out this branch locally
2. Visit an issue and confirm that the space between the "Assignee" header and the first assignee avatar looks like the same distance as that between the "Epic" header and the Epic section's text content. Confirm that the same apparent spacing is in place for the Labels and Participants sections as well.
3. Visit an MR and confirm that the space between the "Assignee" header and the first assignee avatar looks like the same distance as that between the "Epic" header and the Epic section's text content. Confirm that the same apparent spacing is in place for the Reviewers, Labels and Participants sections as well.

Related to #450709 (closed), #425467 (closed)