Use describe on feature tests

The `type: :feature` is automatically detected by RSpec because these
tests are inside the `spec/features` folder. Using `feature` re-adds a
`type: :feature` to these files, which will result in a conflict when we
upgrade to Rails 5.1's system tests.

Because of this change, we also need to change `background` to `before`
or else these tests will fail.

spec/features/admin/stats_spec.rb

@@ -1,8 +1,8 @@
 require "rails_helper"
 
-feature "Stats" do
+describe "Stats" do
 
-  background do
+  before do
     admin = create(:administrator)
     login_as(admin.user)
     visit root_path
@@ -104,7 +104,7 @@ feature "Stats" do
 
   describe "Budget investments" do
     context "Supporting phase" do
-      background do
+      before do
         @budget = create(:budget)
         @group_all_city   = create(:budget_group, budget: @budget)
         @heading_all_city = create(:budget_heading, group: @group_all_city)
@@ -191,7 +191,7 @@ feature "Stats" do
     end
 
     context "Balloting phase" do
-      background do
+      before do
         @budget = create(:budget, :balloting)
         @group = create(:budget_group, budget: @budget)
         @heading = create(:budget_heading, group: @group)