Add an ability to change the jitsi fallback URL

### Description

For private homeservers, creating a video conference in a shared room creates a widget that uses the "https://app.element.io/jitsi.html" webpage which leeches potentially private information (e.g. user details / homeserver details) to a public website.

### Steps to reproduce

- Create a public room
- Add users to that room
- Start a video conference
- Look at the source for the widget

### Describe how what happens differs from what you expected.

The documentation [here](https://github.com/vector-im/element-web/blob/develop/docs/jitsi-dev.md) suggests that a local copy of jitsi.html can be used. I'm not really sure what that means? Local to the homeserver or local to the client itself? I'm not even sure if the latter is possible with Android clients, for example.

What would be really nice is if the "well-known" url can supply this like it can provide (already) the "preferredDomain" value.

### Version information

- **Platform**: desktop, Android, iOS

For the desktop app:

- **OS**: Windows, Linux
- **Version**: 1.7.12