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.