Always assign current user on "Assign myself"

If a username was entered in the assign box and "Assign myself" clicked,
the code would look up the username instead of assigning current user.
The button would only work if no username was entered.

todosrht/blueprints/ticket.py

@@ -300,14 +300,13 @@ def _assignment_get_ticket(owner, name, ticket_id):
     return ticket
 
 def _assignment_get_user(valid):
-    username = valid.optional('username')
-    if not username:
-        if 'myself' in valid:
-            username = current_user.username
-        else:
-            valid.error("Username is required", field="username")
-    if not valid.ok:
-        return None
+    if 'myself' in valid:
+        username = current_user.username
+    else:
+        username = valid.require('username', friendly_name="Username")
+        if not valid.ok:
+            return None
+
     if username.startswith("~"):
         username = username[1:]