Update hashtag prune to account for followed hashtags

Currently pruning hashtags with the prune_objects task only accounts
for whether that hashtag is associated with an object, but this may
lead to a foreign key constraint violation if that hashtag has no
objects but is followed by a local user.

This adds an additional check to see if that hashtag has any followers
before proceeding to delete it.

lib/mix/tasks/pleroma/database.ex

@@ -346,7 +346,10 @@ def run(["prune_objects" | args]) do
       DELETE FROM hashtags AS ht
       WHERE NOT EXISTS (
         SELECT 1 FROM hashtags_objects hto
-        WHERE ht.id = hto.hashtag_id)
+        WHERE ht.id = hto.hashtag_id
+        UNION
+        SELECT 1 FROM user_follows_hashtag ufht
+        WHERE ht.id = ufht.hashtag_id)
       """
       |> Repo.query!()