Delete posts using `whereIn`

Delete checked posts using eloquent `whereIn`, invalid ids are
automatically escaped.
Disable the delete button on success
Update required permissions

- Added requiredPermissions to `posts` and `categories` controllers.
- Added user_id to database table.
- Created a new permission to allow user to manage others posts, and
restrict user to manage only their posts.
Merge pull request #40 from Whyounes/pull-request

Delete posts using `whereIn`