[FE] Design Notes should also be available in the discussions tab

This is a placeholder for frontend related work for #11851 (closed)

Proposal

Similar to #12528 (closed), add a system note when someone adds a comment on a design. Then, link to the comment in that design.

Mockup

Note the copy in the system note has changed since this mockup. The new copy is {user} started a discussion on {design filename} #11851 (comment 243483611).