Add CKEditor usage on content fields

[sveetch]

If django-ckeditor is installed, admin forms should enable CKEditor usage on some fields like Category.description, Article.introduction and Article.content. But not Article.lead which is used for meta information, so keep it in plain text.

[sveetch]

For now, this is not optional and django-ckeditor is in base requirements.

In future CKEditor usage should be moved to a contrib application with some mechanics so it could be optional and another editor could be used instead with another contrib app.