Allow full stops in SMS senders

We have a team who want their (short) web address as the text message
sender. This commit updates the validation of text message senders to
allow `.` as a valid character, which is currently blocking them from
doing this.

We can be fairly confident this works because:

- the team are sending large volumes of messages already with their
  existing provider
- we’ve tested it with all combinations of
  - both our text message providers
  - an Android phone and n iPhone

tests/app/main/views/test_service_settings.py

@@ -814,7 +814,7 @@ def test_incorrect_letter_contact_block_input(
     ('', 'Can’t be empty'),
     ('abcdefghijkhgkg', 'Enter 11 characters or fewer'),
     (' ¯\_(ツ)_/¯ ', 'Use letters and numbers only'),
-    ('blood.co.uk', 'Use letters and numbers only'),
+    ('blood.co.uk', None),
 ])
 def test_incorrect_sms_sender_input(
     sms_sender_input,