Find solution for dealing with non ASCII domains

[fklein-lu]

Currently there's no good way to handle internationalised domains that use non ASCII characters. Characters can either be displayed, or encoded characters might be used.

We should align on a safe, reliable storage mechanism. One possible direction to investigate would be to use Core's Requests_IDNAEncoder class.

[fklein-lu]

After talking to @rmccue, we decided to use the Internationalized Domain Names in Applications (IDNA) Protocol, as outlined in RFC 5891.

Storage

Both the From and To URLs will be stored as absolute URLs in IDNA encoded format. Redirect matching will be done via IDNA encoded URLs as well.

Options for implementing the IDNA encoding:

• Use the Requests_IDNAEncoder class in WordPress.
• Use the true/punycode Composer package.

UI

The UI should display the decoded URLs, to reduce confusion among users. But the UI should also offer a way to view the IDNA encoded variation.