gluon-luci-theme: add cbi-value-field-long class

This new class allows visually pleasing lists of radio buttons/checkboxes/etc.
with long explanation texts.

package/gluon-luci-theme/sass/cascade.scss

@@ -648,6 +648,21 @@ div.cbi-value-field {
   }
 }
 
+div.cbi-value-field-long {
+  flex: 10;
+  position: relative;
+
+
+  input, select, input + label {
+    position: relative;
+    top: -0.39em;
+  }
+}
+
+div.cbi-value-field-long-after {
+  flex: 2;
+}
+
 div.cbi-value-description {
   font-size: 8pt;
 }