Allow nuking cross signing in RiotX

When I first tried to bootstrap cross signing, I used a previous version of RiotX, but it crashed during the process.

The state now is that RiotX thinks that cross signing is active, but Riot Web thinks it's not. Trying to sign other devices from RiotX doesn't work. See https://github.com/vector-im/riot-web/issues/13611 for details.

It seems that Riot Web offers the possibility to nuke cross signing in case something goes badly wrong, but it doesn't give me this option because it thinks that cross signing hasn't been bootstrapped.

Please provide this possibility in RiotX, too, so that I (and at least two other users who have the same problem) can finally setup cross signing.