Force user to enter Security Key at end of cross signing setup

### Your use case

#### What would you like to do?

When cross signing is setup for the first time the user is presented with a security key. Right now Element just tells you to copy it and/or save it to a file but it doesn't check that you have actually saved it. After presenting the Security Key, Element should immediately ask the user to type it in to prove they have in fact saved it.

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

A very common support request is people asking how they find their security key. It seems like people are being presented the security key during cross signing setup but not actually saving it.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_