Replace `render "public/404.html"` with `render file: "public/404.html"`

This change is backward-compatible, so it shouldn't break existing any
existing app.

Previously, calling render "foo/bar" in a controller action was
equivalent to render file: "foo/bar". In Rails 4.2, this has been
changed to mean render template: "foo/bar" instead.

See http://guides.rubyonrails.org/4_2_release_notes.html#render-with-a-string-argument
Rename ArticleController to ArticlesController

Because the latter follows Rails' convetions.
Also, there is no need to implement override_authority in
ArticleController, since instance variable name `@article` follows the
convetion.
Removing `override_authority` from the controller, however, breaks the
tests since https://github.com/brentgreeff/fat_model_auth/commit/60e8d38f#diff-a583a86a8141e7bfa5889bcf2bb5d615L33
introduces a bug caused by missing `.singularize`.
This bug popped up in a real app when I tried to upgrade fat_model_auth.
There is no need to use action_name as authority_name

This behavior was added in
https://github.com/brentgreeff/fat_model_auth/commit/60e8d38f#diff-a583a86a8141e7bfa5889bcf2bb5d615L33
for no good reason, let's remove it.
Merge pull request #2 from DNNX/master

Replace `render "public/404.html"` with `render file: "public/404.html"`