Sort letter brandings alphabetically

Currently the brandings have non-deterministic sorting, which means
the order changes from page load to page load. This makes it hard to
find the item you’re looking for.

This commit sorts them by the name of the branding, same as for email
brandings.

app/main/forms.py

@@ -1,6 +1,7 @@
 import weakref
 from datetime import datetime, timedelta
 from itertools import chain
+from operator import itemgetter
 
 import pytz
 from flask_wtf import FlaskForm as Form
@@ -785,7 +786,10 @@ class LetterBranding(StripWhitespaceForm):
 
     def __init__(self, choices=[], *args, **kwargs):
         super().__init__(*args, **kwargs)
-        self.dvla_org_id.choices = choices
+        self.dvla_org_id.choices = list(sorted(
+            choices,
+            key=itemgetter(1),
+        ))
 
     dvla_org_id = RadioField(
         'Which logo should this service’s letter have?',