Project: Fix server error while deleting

Fix a bug that a referential integiry exception occurrs while deleting
a project which has an issue to which a label is attached. Delete issues
first because labels cannot be deleted if there is an issue refers them.

app/models/Project.java

@@ -609,14 +609,16 @@ public void delete() {
             label.update();
         }
 
-        for(IssueLabelCategory category : IssueLabelCategory.findByProject(this)) {
-            category.delete();
-        }
-
+        // Issues must be deleted before issue labels because issues may refer
+        // issue labels.
         for(Issue issue : issues) {
             issue.delete();
         }
 
+        for(IssueLabelCategory category : IssueLabelCategory.findByProject(this)) {
+            category.delete();
+        }
+
         for (Assignee assignee : assignees) {
             assignee.delete();
         }