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/assets/javascripts/html_editor.js

@@ -0,0 +1,14 @@
+(function() {
+  "use strict";
+  App.HTMLEditor = {
+    initialize: function() {
+      $("textarea.html-area").each(function() {
+        if ($(this).hasClass("admin")) {
+          CKEDITOR.replace(this.name, { language: $("html").attr("lang"), toolbar: "admin", height: 500 });
+        } else {
+          CKEDITOR.replace(this.name, { language: $("html").attr("lang") });
+        }
+      });
+    }
+  };
+}).call(this);