Merge branch 'dz-handle-unmatched-routes' into 'master'

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.

See merge request !6905