Swap all `idna::domain_to_ascii` to `idna::domain_to_ascii_cow`

[`idna::domain_to_ascii` docs](https://docs.rs/idna/latest/idna/fn.domain_to_ascii.html) say:

> This function exists for backward-compatibility. Consider using [`domain_to_ascii_cow`](https://docs.rs/idna/latest/idna/fn.domain_to_ascii_cow.html) instead.

In the process, an `idna::AsciiDenyList` argument will be added. `EMPTY` would retain the previous behavior; `URL` may be preferred in some cases.

Search the repository for every use of idna::domain_to_ascii, then read the linked idna::domain_to_ascii and idna::domain_to_ascii_cow documentation. Update each call with the appropriate AsciiDenyList choice, using EMPTY to retain previous behavior where applicable, and run the existing test suite to verify the replacements.

Written by the indexing model from the issue text.