Make Internationalized domains searchable

### Your use case

#### What would you like to do?
I would like to easily search for users on servers with internationalized domains. When I enter `@hans:bücher.de` I want to find Hans on the `Bücher.de` instance, instead I have to type something like `@hans:xn--bcher-kva.de`. Currently by using the internationalized domain name you find nothing.

#### Why would you like to do it?
It's straight forward to use the domain name in human readable language instead of an encoded alias like `xn--bcher-kva.de`. It's less error prone and looks less suspicious, after all `bücher.de` is what you use to log-in to the matrix server, its what you type into the address bar of the browser so why would it be different in Element (or Matrix)?

#### How would you like to achieve it?
By entering the internationalized domain name when searching for a user.

This is related to #1988