Allow mobile redirect to hydrogen

### Your use case

#### What would you like to do?
When a user visits the element-web interface from a mobile device they get the "please install an app or use the desktop site" message which is great.

However, if the server administrator has hydrogen installed the interface could offer that as an option provided it's set in the config.

#### Why would you like to do it?
Offer users more choices

#### How would you like to achieve it?
In hydrogen-web's config.json the following options:
```
offer_hydrogen_interface = true (obviously off by default)
hydrogen_url = https://hydrogen.mydomain.com
```

### Have you considered any alternatives?

I've considered altering the code on my own server, however I think it would be a great feature to be kept upstream.

### Additional context

_No response_

Start by locating the mobile-device message and the hydrogen-web config.json handling in element-web. Verify how configuration values are loaded, then make the mobile interface offer the configured Hydrogen URL only when enabled and preserve the current message and default behavior when it is not configured.

Written by the indexing model from the issue text.