When a sponsor creates a new guest invitation, and when a guest corrects their name
they must now use transcribed characters.

Instead of manually putting together the phone number and hoping we get
it right, we instead format the number using the phonenumbers library.
This library is already in use for validation, making it the obvious
choice. This means that the backend will only receive E.164 formatted
phonenumbers from now on. In addition, we now ensure that the number is
in fact a mobile phone number.

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