Check device signatures of the master key to find verif devices

At the moment when logging in with cross-signing, we hold the user at the "verify this session" / complete security screen suggesting they should use another device to verify.

As an improvement, we could investigate looking for device signatures of the master key and comparing that to active devices as a way to gather whether the user has any other devices than could be used. Then we can customise the instructions to user / alter the UX in case there are no other cross-signing aware devices to use.

**Warning:** If this would add even more requests to the sign in flow (which is already under heavy load), we should consider our approach carefully.