Use unified function to check cross-signing is ready
Fixes mismatches where the Cross signing panel would say cross
signing was not ready but no toasts would appear.
Fixes https://github.com/vector-im/riot-web/issues/12796
Fixes https://github.com/vector-im/riot-web/issues/12798
Requires https://github.com/matrix-org/matrix-js-sdk/pull/1279
