Make pickable hidden forms elements readonly

Not visible element should be made readonly, since the user can't edit
them anyways.

This fixes a problem where Chrome would auto-fill hidden input and the
value would be saved without the user ever seeing it.

We already tried to fix the problem usign a standard solution
(auto-complete="off", symphonycms#1843 and symphonycms#1841) but it does not work. We also tried a non-standard
solution (symphonycms#2258) which was rejected.

This change only uses standard solution, even thought it relies on
javascript to make things works (it should not be a problem since the
backend now heavily relies on javascript)

symphony/assets/js/src/symphony.pickable.js

@@ -56,12 +56,12 @@
 
 			// Hide all choices
 			object.trigger('pickstart.pickable');
-			related.hide();
+			related.hide().find('input, select, textarea').prop('readonly', true);
 
 			// Selection found
 			picked = $('#' + choice);
 			if(picked.length > 0) {
-				picked.show().trigger('pick.pickable');
+				picked.show().find('input, select, textarea').prop('readonly', false).trigger('pick.pickable');
 				object.trigger('pickstop.pickable');
 			}