Handle unmatched routing with not_found method

What does this MR do?

Handle all unmatched routes with 404 method

Why was this MR needed?

We need this to prevent routing error when user access URL like /123 when there is no resource located under such name.

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/23378 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6733.