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

app/main/forms.py

@@ -35,7 +35,7 @@ from app.main.validators import (
     ValidGovEmail,
     NoCommasInPlaceHolders,
     OnlyGSMCharacters,
-    LettersAndNumbersOnly,
+    LettersNumbersAndFullStopsOnly,
 )
 
 
@@ -560,7 +560,7 @@ class ServiceSmsSenderForm(StripWhitespaceForm):
         validators=[
             DataRequired(message="Can’t be empty"),
             Length(max=11, message="Enter 11 characters or fewer"),
-            LettersAndNumbersOnly(),
+            LettersNumbersAndFullStopsOnly(),
         ]
     )
     is_default = BooleanField("Make this text message sender the default")