Work item comments and threads - editor followups

Follow ups from #389831 (closed), related to the editor

1. Maintain consistent editor placement when switching edit modes

The container for adding/editing a comment changes position when switching Markdown/Rich text modes, with Rich Text maintaining the original placement of the "Add comment" container and Markdown moving down slightly.

2. Pasting a link in the comment box adds a line break

3. Double focus ring on rich text editor

Only the outer container should show the focus ring, mirroring the Markdown editor.