Check for orphaned conversations

[duogenesis]

When XMPP usernames were converted from IDs to UUIDs in May/June, some conversations might've become orphaned from the rest of the account data. Orphaned conversations should be checked for and removed, if they exist.

Before undertaking this ticket, it might be best to wait some time, in case the accounts of the orphaned conversations still exist, and the account holders still want to access the conversations. The conversations will need to be re-linked on a case-by-case basis.