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/site_customization/content_blocks_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-feature "Custom content blocks" do
+describe "Custom content blocks" do
   scenario "top links" do
     create(:site_customization_content_block, name: "top_links", locale: "en",
                                               body: "content for top links")

spec/features/site_customization/custom_pages_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-feature "Custom Pages" do
+describe "Custom Pages" do
 
   context "New custom page" do
     context "Draft" do

spec/features/site_customization/information_texts_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-feature "Custom information texts" do
+describe "Custom information texts" do
 
   scenario "Show custom texts instead of default ones" do
     admin = create(:administrator)