Remove duplication calculating insufficient funds

We were using the same logic twice.

I've moved the logic to the Ballot model, which to me is a more natural
place to calculate whether there's enough money left than the Investment
model. After all, the remaining money is in the ballot, and not in the
investment.

app/models/budget/ballot.rb

@@ -33,6 +33,10 @@ class Budget
       budget.formatted_amount(amount_available(heading))
     end
 
+    def enough_money?(investment)
+      investment.price.to_i <= amount_available(investment.heading)
+    end
+
     def has_lines_in_group?(group)
       groups.include?(group)
     end

app/models/budget/ballot/line.rb

@@ -20,7 +20,7 @@ class Budget
 
       def check_sufficient_funds
         ballot.lock!
-        errors.add(:money, "insufficient funds") if ballot.amount_available(investment.heading) < investment.price.to_i
+        errors.add(:money, "insufficient funds") unless ballot.enough_money?(investment)
       end
 
       def check_valid_heading

app/models/budget/investment.rb

@@ -268,7 +268,7 @@ class Budget
       return :not_selected               unless selected?
       return :no_ballots_allowed         unless budget.balloting?
       return :different_heading_assigned unless ballot.valid_heading?(heading)
-      return :not_enough_money           unless enough_money?(ballot)
+      return :not_enough_money           unless ballot.enough_money?(self)
       return :casted_offline             if ballot.casted_offline?
     end
 
@@ -301,11 +301,6 @@ class Budget
       user.headings_voted_within_group(group).where(id: heading.id).exists?
     end
 
-    def enough_money?(ballot)
-      available_money = ballot.amount_available(heading)
-      price.to_i <= available_money
-    end
-
     def register_selection(user)
       vote_by(voter: user, vote: "yes") if selectable_by?(user)
     end