System notes / Assignee dropdown broken CSS

Summary

As per this conversation: !34360 (comment 359827414)

We have a selection of CSS around system notes and the assignee dropdowns that need to be fixed:

1. System notes alignment => When we go down to mobile, a tiny phantom grey connector line appears below the icon, like it's preparing to accept a second system note that hasn't yet arrived. => !34870 (merged)
2. System notes text wrap, it would be good for the wrapped text to be centered with the icon.
3. Double scroll bars inside the assignee dropdowns