Resolve "MR comment + system note highlight don't have the same width"

What does this MR do?

Fixes inconsistencies in issuable discussion comments and notes (see before/after pics)

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

Play around with comments, notes, diff discussions, and threaded comments in different screen widths to make sure nothing has broken. I've tested this in Safari 10, Chrome 59, and Firefox.

Why was this MR needed?

Issue/MR notes and comments had different widths, leading to inconsistent bottom-borders separators and odd looking highlighting behavior.

Screenshots (if relevant)

before after
before-note-highlight highlighted note after-note-highlight highlighted note
before-comment-highlight highlighted comment after-comment-highlight highlighted comment
before-small-viewport small viewport after-small-viewport small viewport
before-mobile-viewport mobile viewport after-mobile-viewport mobile viewport

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #33917 (closed)

Edited by username-removed-636429