Allow platform admins to change daily message limit

One less thing we have to go into the database to do, and remember to
manually clear the cache for after.

app/main/forms.py

@@ -1174,6 +1174,15 @@ class FreeSMSAllowance(StripWhitespaceForm):
     )
 
 
+class MessageLimit(StripWhitespaceForm):
+    message_limit = GovukIntegerField(
+        'Number of messages the service is allowed to send each day',
+        validators=[
+            DataRequired(message='Cannot be empty')
+        ]
+    )
+
+
 class ConfirmPasswordForm(StripWhitespaceForm):
     def __init__(self, validate_password_func, *args, **kwargs):
         self.validate_password_func = validate_password_func