css: Don't apply .collapsible-control style to custom controls

public/css/icinga/main.less

@@ -304,8 +304,11 @@ a:hover > .icon-cancel {
 // Collapsibles
 
 .collapsible.collapsed {
-  position: relative;
   overflow: hidden;
+}
+
+.collapsible.collapsed:not([data-toggle-element]) {
+  position: relative;
 
   &:before, &:after {
     content: "";
@@ -328,7 +331,7 @@ a:hover > .icon-cancel {
   }
 }
 
-.impact .collapsible.collapsed {
+.impact .collapsible.collapsed:not([data-toggle-element]) {
   &:before {
     opacity: 0;
   }

public/css/icinga/widgets.less

@@ -235,10 +235,6 @@ form.role-form {
     padding: 0;
     margin: 0;
 
-    &:before {
-      display: none;
-    }
-
     h3 em {
       font-size: .857em;
       font-weight: normal;