Make priority field type NOT changeable

This is necessary because the system depends ticket priority to sort and
display ticket data.

include/class.dynamic_forms.php

@@ -54,10 +54,13 @@ class DynamicForm extends VerySimpleModel {
     }
 
     function getDynamicFields() {
-        if (!isset($this->_dfields))
+        if (!isset($this->_dfields)) {
             $this->_dfields = DynamicFormField::objects()
                 ->filter(array('form_id'=>$this->id))
                 ->all();
+            foreach ($this->_dfields as $f)
+                $f->setForm($this);
+        }
         return $this->_dfields;
     }
 

include/class.forms.php

@@ -798,6 +798,10 @@ class PriorityField extends ChoiceField {
     function hasIdValue() {
         return true;
     }
+    function isChangeable() {
+        return $this->getForm()->get('type') != 'T' ||
+            $this->get('name') != 'priority';
+    }
 
     function getChoices() {
         global $cfg;