Add admin UI for SMS cost-restricted dial codes

What does this MR do and why?

Add admin UI for SMS cost-restricted dial codes

The cost-restricted dial code list could only be set through the API, which made it hard to see which countries were affected. Admins now get a checkbox list under New user account restrictions.

Several countries share a dial code (+1 covers 26 of them), so each checkbox is a dial code with every country on it listed, making clear what restricting a code affects. Codes saved without a matching supported country still render so saving the form does not drop them.

The form submits an array with a blank placeholder so clearing every checkbox empties the list; the writer drops blanks before casting.

Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/609536

Changelog: added EE: true

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Related to #609536