Previously in Polygerrit, comments were always expanded. You could
always see the full comment (if multiline) and any applicable actions.
This change creates a collapsed comment view. It adds a preview of the
text to the header row when collapsed, and can be toggled open when any
part of the header is clicked.
Bug: Issue 4698
Change-Id: Idca5caf92eb32518b6737dbb5a3380d227513996