Giant-news

- Add `_default_author()` method to retrieve the first default author
- Allow Authors to be deleted without CASCADING all of the related articles. Achieved by providing
a default_author method to SET_DEFAULT on the author foreignkey.
- Add `is_default` to Author model and expose in the admin

- Add `name` as searchable field for TagAdmin

- Don't register CMS app by default. Controlled through settings variable REGISTER_NEWS_APP.

- Don't register admin by default (added steps to README)
- Fix template syntax so it doesn't raise Invalid block tag
- Fix `verbose_name_plural` for Category model
- Change Placeholder name to `class-name_placeholder` for generic content

- Refactor models.py into a directory
- Removed customisation options for admin classes. Inheritance is now the
recommended customisation option
- Introduced AbsractArticle class and Article class to allow for more
flexibility in the future. Article is now a swappable model.
- Reset migrations
- Fixed current article being displayed on the related articles plugin

- Add sitemap