Fix behavior of mobile receiving when first entering a quantity

i.e. replace default of '1' if user first presses a numeric key, but
always append to quantity after that

tailbone/static/js/tailbone.mobile.js

@@ -111,23 +111,37 @@ $(document).on('click', '#receiving-quantity-keypad-thingy .keypad-button', func
     var quantity = $('.receiving-quantity');
     var value = quantity.text();
     var key = $(this).text();
+    var changed = $('#receiving-quantity-keypad-thingy').data('changed');
     if (key == 'Del') {
         if (value.length == 1) {
             quantity.text('0');
         } else {
             quantity.text(value.substring(0, value.length - 1));
         }
+        changed = true;
     } else if (key == '.') {
         if (value.indexOf('.') == -1) {
+            if (changed) {
                 quantity.text(value + '.');
+            } else {
+                quantity.text('0.');
+                changed = true;
+            }
         }
     } else {
         if (value == '0') {
             quantity.text(key);
-        } else {
+            changed = true;
+        } else if (changed) {
             quantity.text(value + key);
+        } else {
+            quantity.text(key);
+            changed = true;
         }
     }
+    if (changed) {
+        $('#receiving-quantity-keypad-thingy').data('changed', true);
+    }
 });
 
 

tailbone/templates/mobile/receiving/view_row.mako

@@ -38,7 +38,7 @@
   </tbody>
 </table>
 
-<table id="receiving-quantity-keypad-thingy">
+<table id="receiving-quantity-keypad-thingy" data-changed="false">
   <tbody>
     <tr>
       <td>${h.link_to("7", '#', class_='keypad-button ui-btn ui-btn-inline ui-corner-all')}</td>