Add support for international domain names

[DL6ER]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)

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What does this PR aim to accomplish?:

Add support for international domain names.

How does this PR accomplish the above?:

Convert domains to IDN form (if applicable) before adding them to the database.
Note that this relies on php-intl to be installed, I will add it in a separate PR to the core repo.

Traditional domains list

Group management domains page

What documentation changes (if any) are needed to support this PR?:

None

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/url-allestorungen/26659/19

[DL6ER]

For those who want to test it but don't have Umlauts on their keyboards, you can copy&paste the test domains from below:

allestörungen.de
testäää.de

[DL6ER]

The necessary core PR has been merged.

[DL6ER]

This may be reviewed and merged at any time.

[dschaper]

Does this require the v2 build containers or the libidn things?

[DL6ER]

No, it's only using PHP stuff which we already added in pi-hole/pi-hole#3076

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/database-error-after-update/27966/6