Use acts_as_taggable for investment valuation tags

We were manually doing the same thing, generating inconsistent results,
since the method `valuation_tag_list` was using the `valuation` context,
when actually the expected behavior would be to use the `valuation_tag`
context.

app/controllers/admin/budget_investments_controller.rb

@@ -110,7 +110,7 @@ class Admin::BudgetInvestmentsController < Admin::BaseController
     end
 
     def load_tags
-      @tags = Budget::Investment.tags_on(:valuation).order(:name).distinct
+      @tags = Budget::Investment.tags_on(:valuation_tags).order(:name).distinct
     end
 
     def load_ballot