BUG Take first non-empty link field for preview

Goes by priority, but allows for fields to be empty.
This is important e.g. for models which are removed just
from live, or just from draft.
Also reacts to a field rename done in CMS module (62783c7).

admin/javascript/LeftAndMain.Preview.js

@@ -67,7 +67,10 @@
 				if(this.is('.is-collapsed')) return;
 
 				// var url = ui.xmlhttp.getResponseHeader('x-frontend-url');
-				var url = $('.cms-edit-form').find(':input[name=PreviewURL],:input[name=StageURLSegment]').val();
+				var url = $('.cms-edit-form')
+					.find(':input[name=PreviewURL],:input[name=StageLink],:input[name=LiveLink]')
+					.filter(function() {return $(this).val() !== '';})
+					.val();
 				if(url) {
 					this.loadUrl(url);
 					this.unblock();
@@ -294,7 +297,11 @@
 			onclick: function(e) {
 				e.preventDefault();
 
-				var preview = $('.cms-preview'), url = $('.cms-edit-form').find(':input[name=PreviewURL],:input[name=StageURLSegment]').val();
+				var preview = $('.cms-preview'), 
+					url = $('.cms-edit-form')
+						.find(':input[name=PreviewURL],:input[name=StageLink],:input[name=LiveLink]')
+						.filter(function() {return $(this).val() !== '';})
+						.val();
 				if(url) {
 						preview.loadUrl(url);
 						preview.unblock();