Who does Orwell suggest is responsible for spreading the idea that Snowball visits the farm at night?

It's Squealer: ever since Snowball was driven from the farm, Napoleon's propagandist-in-chief has never lost the opportunity to trash Snowball's reputation, making him out to be a traitor to the Animalist Revolution. It's Squealer who turns Snowball into a scapegoat for the many problems that have befallen the farm, most of which were due to Napoleon's incompetence.
The collapse of the windmill is a case in point; it was so poorly constructed that it was blown down quite easily in a storm. Napoleon immediately heaps the blame for the windmill's collapse on the absent Snowball. The ever-loyal Squealer sees this as an great opportunity to turn Snowball into a boogie-man figure, someone who can conveniently be blamed for the regime's numerous failings.
With a completely straight face, Squealer accuses Snowball of regularly sneaking back onto the farm in the dead of night and committing acts of sabotage. Apparently, this is because he's formed a treacherous alliance with the hated human oppressors and is determined to do anything he can to undermine the Revolution and make himself ruler of the farm with the help of the enemy's assistance.