aba6f5aafc
By not implementing MetaMerger, UserLogin metadata such as BridgedGuildIDs would get clobbered upon reauthing because the metadata was replaced entirely. Implement CopyFrom so we can gain deeper control over what is preserved upon reauth. Notably, preserve BridgedGuildIDs so we can't get into a weird state where a guild is bridged but simultaneously absent from BridgedGuildIDs, which would cause us to not subscribe (OP 14) to it properly.