Attempt to fix megolm key not being in SSSS
If the account has both a key backup and SSSS enabled but the key
backup key isn't stored in SSSS, try to save it there by getting
it from either the cache or the user (if perhaps it's the same as
their security passphrase but lost a passthrough entry).
Fixes https://github.com/vector-im/element-web/issues/17886
