Refactor to use validator class

Using a separate validator class to check for appropriate characters in
a text message sender means that we’re not doing this validation in a
different way from the other checks (length and required). So the code
is cleaner.

tests/app/main/views/test_service_settings.py

@@ -810,7 +810,8 @@ def test_incorrect_letter_contact_block_input(
 
 @pytest.mark.parametrize('sms_sender_input, expected_error', [
     ('', 'Can’t be empty'),
-    ('abcdefghijkhgkg', 'Enter 11 characters or fewer')
+    ('abcdefghijkhgkg', 'Enter 11 characters or fewer'),
+    (' ¯\_(ツ)_/¯ ', 'Use letters and numbers only'),
 ])
 def test_incorrect_sms_sender_input(
     sms_sender_input,