In f8bcf39014, we fixed buggy
marshalling of Streams and similar data structures where we were
including the Stream object rather than its ID in dictionaries passed
to ujson, and ujson happily wrote that large object dump into the
RealmAuditLog.extra_data field.
This commit includes a migration to fix those corrupted RealmAuditLog
entries, and because the migration loop is the same, also fixes the
format of similar RealmAuditLog entries to be in a more natural format
that doesn't weirdly nest and duplicate the "property" field.
Fixes#16066.
Anders Kaseorg
Tim Abbott