Renaming discourse_voting to topic_voting since there are two forms of voting in Discourse - posts and topics.
This PR also moves a OnceOff into a post migration. The post migration will be executed, but should ideally be a no-op. This allows us to not have to maintain the OnceOff as it had to be modified before with a previous migration. I considered removing this file altogether, but I don't think there is anything negative from just converting it into a migration, and it might be useful in the unlikely scenario that a forum from the past has never ran the OnceOff before.