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/validators.py

@@ -66,9 +66,9 @@ class OnlyGSMCharacters:
             )
 
 
-class LettersAndNumbersOnly:
+class LettersNumbersAndFullStopsOnly:
 
-    regex = re.compile(r'^[a-zA-Z0-9\s]+$')
+    regex = re.compile(r'^[a-zA-Z0-9\s\.]+$')
 
     def __init__(self, message='Use letters and numbers only'):
         self.message = message