Merge branch 'dz-refactor-project-routes' into 'master'

Refactor project routing

## What does this MR do?

Refactor project routing: 

* split on multiple files
* improve routing order


## Why was this MR needed?

It makes it easier to maintain and modify project routing

## What are the relevant issue numbers?

Extracted from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7121

See merge request !7329