Allow linking devices from Element X to Element X

### Your use case

#### What would you like to do?

Use one session in Element X to create a new session in another Element X.

#### Why would you like to do it?

- Using multiple phones in parallel
- Using a tablet and a phone in parallel
- Migration of phone/tablet to another phone/tablet

#### How would you like to achieve it?

Similar to the current Link new device in Element Desktop. In the logged-in Element X, select Link new device. A QR code is presented. Scan the QR code with the Element X that is not yet logged in – this part of the flow is already implemented. The new device will be fully set up, including cross signing and backup.

Optional: It could be possible that one of the devices doesn't have a camera, or the camera could be more cumbersome to use (e.g. tablet). Figure out (automatically, maybe assisted by user choice) which of the two devices should actually present the QR code and which should scan. But this is really optional for now.

### Have you considered any alternatives?

Logging in, and setting up backup manually is not user friendly.