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/users_spec.rb

@@ -1,7 +1,7 @@
 require "rails_helper"
 
-feature "Admin users" do
-  background do
+describe "Admin users" do
+  before do
     @admin = create(:administrator)
     @user  = create(:user, username: "Jose Luis Balbin")
     login_as(@admin.user)