Avatars and system icons on left of notes are shown on mobile

What does this MR do?

Makes avatars and system icons on discussion notes hidden on mobile.

Are there points in the code the reviewer needs to double check?

Check if map-get($grid-breakpoints, sm) = 0.

Why was this MR needed?

To fix design broken on mobile.

Screenshots (if relevant)

mobile

before	after

PC (unchanged)

before	after

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• Tests passed for this feature/bug
• Conform by the code review guidelines
  • [n/a] Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • [n/a] Has been reviewed by a Backend maintainer
  • [n/a] Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• [n/a] Internationalization required/considered
• [n/a] End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #47604 (closed)