Ensure key backup gets dealt with correctly during secret storage reset
Fixes https://github.com/vector-im/riot-web/issues/13562
We only initialize a new key backup if the user requested one. If they've requested new keys but have not asked for keys to be backed up, we simply delete the now-invalid backup.
This also adds some logging to identify in rageshakes when someone resets their cross-signing, and when their key backup is being deleted.