International domain names are not recognized in mentions

**Describe the bug**
International domain names like https://bawü.social/ are not currently recognized as valid in mentions. As this example illustrates, such domain names are in fact used for at least one prominent Mastodon instance, so any such instances are effectively cut off from @ mentions when using Phanpy (you can use the Punycode representation, but few people will know how to do that).

**Phanpy instance**
- phanpy.social

**To Reproduce**
Steps to reproduce the behavior:
1. Go to phanpy.social and authenticate with your instance
2. Compose a new post
3. Copy and paste this username: @stadtpforzheim@bawü.social

**Expected behavior**
This is a valid username, so it should be fully completed for purposes of an @ mention.

**Screenshots**

**Desktop (please complete the following information):**
- OS: Ubuntu 20.04
- Browser: Firefox
- Version: 120.0.1

**Additional context**

- See https://en.wikipedia.org/wiki/Internationalized_domain_name for more details about international domain names.
- The Mastodon web UI handles this test case correctly.