In the last commit, I introduced a topic_custom_field to determine if a PM is indeed a bot PM.
This commit adds a migration to backfill any PM that is between 1 real user, and 1 bot. The correct topic_custom_field is added for these, so they will appear on the bot conversation sidebar properly.
We can also drop the joining to topic_users in the controller for sidebar conversations, and the isPostFromAiBot logic from the sidebar.
