FEATURE: Add a plugin callback when editing a post

FIX: Wrap PostRevisor database work in a transaction

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I feel uneasy about having events that plugins can listen to without a hard schema and documentation for them (the events). [Bukkit/CraftBukkit](/Bukkit/Bukkit) get that for free with Java's type safety and javadocs, but with the DiscourseEvent class there's not really an obvious place to put documentation.