Bring back CKEditor images button

It was accidentally deleted in commit 914bfa6.

Note the following spec passes on my machine if we add a `sleep 0.1`
call in the `:wait_readable` part of ruby's `Net::Protocol#rbuf_fill`.
Otherwise, it hangs forever after clicking the `.fileupload-file` div,
which closes its window. It might be solved when upgrading rails,
capybara, selenium or chromedriver.

scenario "Allows images in CKEditor", :js do
  visit edit_admin_site_customization_page_path(custom_page)

  within(".ckeditor") do
    within_frame(0) { expect(page).not_to have_css("img") }

    expect(page).to have_css(".cke_toolbar .cke_button__image_icon")
    find(".cke_toolbar .cke_button__image_icon").click
  end

  within_window(window_opened_by { click_link "Browse Server" }) do
    attach_file :file,
                Rails.root.join('spec/fixtures/files/clippy.jpg'),
                visible: false
    find(".fileupload-file").click
  end

  click_link "OK"

  within(".ckeditor") do
    within_frame(0) { expect(page).to have_css("img") }
  end
end

app/views/admin/site_customization/pages/_form.html.erb

@@ -41,7 +41,8 @@
     <%= f.translatable_text_field :title %>
     <%= f.translatable_text_field :subtitle %>
     <div class="ckeditor">
-      <%= f.translatable_cktext_area :content %>
+      <%= f.translatable_cktext_area :content,
+                                     ckeditor: { language: I18n.locale, toolbar: "admin" } %>
     </div>
     <div class="small-12 medium-6 large-3 margin-top">
       <%= f.submit class: "button success expanded" %>

spec/features/admin/site_customization/pages_spec.rb

@@ -41,8 +41,11 @@ feature "Admin custom pages" do
   end
 
   context "Update" do
-    scenario "Valid custom page" do
+    let!(:custom_page) do
       create(:site_customization_page, title: "An example custom page", slug: "custom-example-page")
+    end
+
+    scenario "Valid custom page" do
       visit admin_root_path
 
       within("#side_menu") do
@@ -62,6 +65,14 @@ feature "Admin custom pages" do
       expect(page).to have_content "Another example custom page"
       expect(page).to have_content "another-custom-example-page"
     end
+
+    scenario "Allows images in CKEditor", :js do
+      visit edit_admin_site_customization_page_path(custom_page)
+
+      within(".ckeditor") do
+        expect(page).to have_css(".cke_toolbar .cke_button__image_icon")
+      end
+    end
   end
 
   scenario "Delete" do