Allows services to choose if they can send letters

Our support ticket analysis shows that the most common action request
after going live is turning on letters.

We just do this for any team that requests it – there’s no gatekeeping.
So we should just allow people to make the change themselves.

This will be a better experience for our users, and less work for us.
The design of the page replicates roughly what we have for international
text messaging.

app/main/forms.py

@@ -584,6 +584,17 @@ class ServiceLetterContactBlockForm(StripWhitespaceForm):
             )
 
 
+class ServiceSwitchLettersForm(StripWhitespaceForm):
+
+    enabled = RadioField(
+        'Send letters',
+        choices=[
+            ('on', 'On'),
+            ('off', 'Off'),
+        ],
+    )
+
+
 class ServiceBrandingOrg(StripWhitespaceForm):
 
     def __init__(self, organisations=[], *args, **kwargs):