Sort selected item to the top

When you land on the page it’s good to be able to quickly see what the
currently-set value is, before you change it.

This is unnecessarily hard if the selected item is buried half way down
the page. This commit moves it to the top.

app/main/forms.py

@@ -788,6 +788,7 @@ class LetterBranding(StripWhitespaceForm):
         self.dvla_org_id.choices = list(sorted(
             choices,
             key=lambda choice: (
+                choice[0] != kwargs.get('dvla_org_id'),
                 choice[0] != '001',
                 choice[1],
             ),