The users did not understand what to do when shown the default error
report box telling them that their information was already registered.
This commit shows a less "scary" box with less text, telling them what
to do in addition to what the problem is. Hopefully that helps.

This can be removed in the future when the backend design allows us to
save this information so that the hosts can make a decision for these
cases.

Resolves: GREG-324

- Add link to clearify why it's important
- Change name of button and header in pop-up

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.

Enable us to render a spinner while the hook loads