Don't allow users to modify international code

[jmhooper]

Why: So that user's will not accidentally change it while typing
their phone number

Also note that there is a change in here to allow users to continue
typing a phone number, even if libphonenumber thinks the number is
invalid. This is there to address reported issues where users could not
type their phone number into the phone number input.

Hi! Before submitting your PR for review, and/or before merging it, please
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• When adding a new controller that requires the user to be fully
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[monfresh]

Did you mean INTERNATIONAL?

[monfresh]

Do we have a specific example we can write a test for that shows that you can type in a number that libphonenumber thinks is invalid?

[monfresh]

LGTM % comments

[jmhooper]

Do we have a specific example we can write a test for that shows that you can type in a number that libphonenumber thinks is invalid?

Yes, a US phone number with more than 10 digits. I just added a test to cover that.