Set min text message sender length to 4 characters

We’ve learned of a change implemented today by the UK mobile
network operators, to stop allowing text message sender names
of 3 or less characters.

Adding this validation will not affect existing senders, only those
users trying to add to or update their senders.

app/main/forms.py

@@ -644,7 +644,7 @@ class ServiceSmsSenderForm(StripWhitespaceForm):
         validators=[
             DataRequired(message="Can’t be empty"),
             Length(max=11, message="Enter 11 characters or fewer"),
-            Length(min=3, message="Enter 3 characters or more"),
+            Length(min=4, message="Enter 4 characters or more"),
             LettersNumbersAndFullStopsOnly(),
             DoesNotStartWithDoubleZero(),
         ]