Align collapsed sidebar avatar container

What does this MR do?

Aligns collapsed sidebar avatar container. More specifically, this aligns to the center the avatar icon containing the project or group icon, when the left sidebar is collapsed.

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides