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