Add upward and downward in

Add upward & downward method and routing into Category and Topic.
Add two relevant routing tests in.
Add destroy method for privileges

When a category is destroyed, relevant privileges will also be
destroyed.
Add two factories for tests.
Add validate for articles and posts.
Add a wrap-up function for before_destroy hook.
Add tests to User

Add relationship tests to User
Add privileges deletion to User
Add related tests to User