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/emails/emails_download_spec.rb

@@ -1,10 +1,10 @@
 require "rails_helper"
 
-feature "Admin download user emails" do
+describe "Admin download user emails" do
 
   let(:admin_user) { create(:user, newsletter: false, email: "admin@consul.dev") }
 
-  background do
+  before do
     create(:administrator, user: admin_user)
     login_as(admin_user)
   end

spec/features/admin/emails/newsletters_spec.rb

@@ -1,8 +1,8 @@
 require "rails_helper"
 
-feature "Admin newsletter emails" do
+describe "Admin newsletter emails" do
 
-  background do
+  before do
     admin = create(:administrator)
     login_as(admin.user)
     create(:budget)