When updating the position of a category, the server correctly updates the position in the database, but the response sent back to the client still contains the old position, causing it to "flip back" in the UI when saving. Only reloading the page will reveal the new, correct value.
The Positionable concern correctly positions the record and updates the database, but we don't assign the new position to the already instantiated model.
This change just assigns self.position after the database update. 😎
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