Fix circle icon size calculation in less (#156)

Author: maximthomas

ui/commons/src/main/resources/css/common/helpers.less

@@ -141,7 +141,7 @@ For font awesome
     height: @diameter;
     border-radius: 50%;
     line-height: @diameter;
-    font-size: @diameter/2;
+    font-size: (@diameter/2);
     display: table;
     text-align: center;
     overflow: hidden;