Instead of twisting `AuthBody`, this adds a new component for the different
styling of post-auth security flows. This also makes them fixed width and
adjusts padding to match designs.
This adds a step after login to complete security for your new session. At the
moment, the only verification method is entering your SSSS passphrase, but nicer
paths will be added soon.
This new step only appears when crypto is available and the account has
cross-signing enabled in SSSS.
Fixes https://github.com/vector-im/riot-web/issues/11214