handle renaming in pre_save

I chose pre_save so that if renaming fails, the db and disk remain consistent.

ttadmin/users/models.py

@@ -200,11 +200,16 @@ def on_townie_pre_save(sender, instance, **kwargs):
 
     existing = existing[0]
 
-    if not existing.reviewed and instance.reviewed == True:
+    # See if we need to create this user on disk.
+    if not existing.reviewed and instance.reviewed is True:
         instance.create_on_disk()
         instance.send_welcome_email()
         instance.write_authorized_keys()
 
+    # See if this user needs a rename on disk
+    if existing.username != instance.username:
+        instance.rename_on_disk(existing.username)
+
 
 def _guarded_run(cmd_args, **run_args):
     """Given a list of args representing a command invocation as well as var