Fix #1651 Missing responsive width variables in form

src/definitions/collections/form.less

@@ -668,7 +668,7 @@
 }
 
 /* Swap to full width on mobile */
-@media only screen and (max-width : 767px) {
+@media only screen and (max-width : @largestMobileScreen) {
   .ui.form .two.fields > .fields,
   .ui.form .two.fields > .field,
   .ui.form .three.fields > .fields,
@@ -766,7 +766,7 @@
 }
 
 /* Swap to full width on mobile */
-@media only screen and (max-width : 767px) {
+@media only screen and (max-width : @largestMobileScreen) {
   .ui.form .two.fields > .fields,
   .ui.form .two.fields > .field,
   .ui.form .three.fields > .fields,