3.0 pluralize fixtures

This ref #4685 .
After this modification, fixtures will have to be declared in plural form like
```php
	public $fixtures = [
		'app.users',
		'app.organizations',
		'app.articles',
	];
```
instead of the previous singular form
```php
	public $fixtures = [
		'app.user',
		'app.organization',
		'app.article',
	];
```

I also removed all the unnecessary fixtures that were there.
Waiting for the upgrade shell, if this seems good to you I could work tomorrow on updating plugins and documentation before this is been merged