Add budget investments translation interface

* Adapt translatable spec helper method to work with budget investments
* Remove old attributes from strong parameters
* Add missing locales to admin.yml and budgets.yml
* Change SpendingProposal.title_max_length and
  SpendingProposal.description_max_lenght to Budget::Investment methods
* Add budget investment translatable attribute translations

spec/features/admin/budget_investments_spec.rb

@@ -11,6 +11,12 @@ describe "Admin budget investments" do
                   :budget_investment,
                   "admin_budget_budget_investment_path"
 
+  it_behaves_like "translatable",
+                  :budget_investment,
+                  "edit_admin_budget_budget_investment_path",
+                  %w[title],
+                  { "description" => :ckeditor }
+
   before do
     @admin = create(:administrator)
     login_as(@admin.user)