Simplify code to calculate supports

The code is easier to read now, it returns the same results it used to
return, and performance-wise it's probably the same thing, but if it's
not, we'll trust Rails will do optimizations that we don't when we
manually pluck the IDs.

app/models/budget/stats.rb

@@ -130,7 +130,7 @@ class Budget::Stats
     end
 
     def supports(supportable)
-      ActsAsVotable::Vote.where(votable_type: "Budget::Investment", votable_id: supportable.investments.pluck(:id))
+      Vote.where(votable: supportable.investments)
     end
 
     stats_cache(*stats_methods)