Added a validation for a minimum of 3 character.

If the SMS sender is less than 3 characters one of our providers will return an error.

app/main/forms.py

@@ -561,6 +561,7 @@ 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"),
             LettersNumbersAndFullStopsOnly(),
         ]
     )