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/debates_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-feature "Admin debates" do
+describe "Admin debates" do
 
   scenario "Disabled with a feature flag" do
     Setting["process.debates"] = nil
@@ -12,7 +12,7 @@ feature "Admin debates" do
     Setting["process.debates"] = true
   end
 
-  background do
+  before do
     admin = create(:administrator)
     login_as(admin.user)
   end