Added button for cover photo removal for groups

nodebb/nodebb#3832

templates/groups/details.tpl

@@ -3,6 +3,7 @@
 		<div class="controls">
 			<span class="upload"><i class="fa fa-fw fa-4x fa-upload"></i></span>
 			<span class="resize"><i class="fa fa-fw fa-4x fa-arrows"></i></span>
+			<span class="remove"><i class="fa fa-fw fa-4x fa-times"></i></span>
 		</div>
 		<div class="save">[[groups:cover-save]] <i class="fa fa-fw fa-floppy-o"></i></div>
 		<div class="indicator">[[groups:cover-saving]] <i class="fa fa-fw fa-refresh fa-spin"></i></div>