Make the default homeserver for search configurable

### Your use case

#### What would you like to do?
Make the default homeserver for search configurable

#### Why would you like to do it?
For FOSDEM, our users are using the app for attendees.fosdem.org HS, but our rooms are on fosdem.org HS so we want those results to be shown by default as that is where the conference is happening

#### How would you like to achieve it?
Make the search configurable in some way so that it looks like this:

### Have you considered any alternatives?

We patched it downstream last year

### Additional context

_No response_