Use text_area instead of cktext_area

We're going to change CKEditor to an inline editor, and the "ckeditor"
gem doesn't provide an option to do so.

Since using `cktext_area` would automatically generate a "classic"
iframe CKEditor, we need to use `text_area` and load the editor using
JavaScript. Personally I prefer this option anyway.

Note in the jQuery selector we need to use `textarea.html-area`; using
just `.html-area` would fail if there's an error message associated to
the textarea, since Rails will add the `.html-area` class to the error
message.

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

@@ -47,8 +47,7 @@
           <%= translations_form.text_field :subtitle %>
         </div>
         <div class="ckeditor column">
-          <%= translations_form.cktext_area :content,
-                                            ckeditor: { language: I18n.locale, toolbar: "admin" } %>
+          <%= translations_form.text_area :content, class: "html-area admin" %>
         </div>
       <% end %>
     </div>