WARNING: EVIL migration modification after the fact. We can do it because there was no collision when we used the migration so far. Purpose: Make sure that there is no collision between IDs of agentAccounts in migration. Also add polymorphism on IdentityProviderAccount and some pep8 cleaning.
corrected alembic mistakes
Support recursive multipart messages, and add a Source.reprocess_content() method to allow re-saving representations after code changes like this one