Draft: Update activity layout

What does this MR do and why?

• Removes vertical connecting line
• Introduces 13px font size for system notes and sidebar
• Decreases size of system note icon
• Removes vertical space between system notes
• Indents system notes

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Go to a merge request (note: I haven't tested other issuables yet (epics, issues etc)

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.