Add phone region code in install wizard

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Is your feature request related to a problem? Please describe.
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default_phone_region is in the sample config. It says there is no defaults. So I removed it.

Sanity check reported it with a Warning:

Your installation has no default phone region set. This is required to validate phone numbers in the profile settings without a country code. To allow numbers without a country code, please add "default_phone_region" with the respective [ISO 3166-1 code ↗](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2#Officially_assigned_code_elements) of the region to your config file.

So we can't get the green check mark without the setting applied.

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Since it raises a warning, but it can't have a default value (at least the sample told), it deserves a text field in the installation wizard,
with the same link to the wikipdia page

Describe alternatives you've considered
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• Define a default value, but it seems rejected from the comment in the sample config file
• Have a drop-down with the table code, but it seems there is no webservice available for this.

Additional context
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Start by locating the installation wizard and the configuration handling for default_phone_region. Review how the wizard collects settings and how the sanity check reports a missing phone region; done means the wizard offers the region code field with the linked ISO 3166-1 reference and the resulting installation passes the check.