Toggle the display of changes in artifact followup

+ Fix issue when the user tried to clear a comment.

Change-Id: I8653c4fd79acbd7344969c058b7b4680582799f7
Merge remote-tracking branch 'gerrit/5674_hide_changes' into tuleap-stable-master

* gerrit/5674_hide_changes:
  Toggle the display of changes in artifact followup

Conflicts:
	plugins/tracker/www/themes/default/css/style.less

Change-Id: Ie6284095910d1ad67838ce1018f24bd6a9c1fde5
Tuleap 6.8.99.10

Change-Id: Ib9d0af7ff7f5f46abaed0d353a1a13af325e67e1