Merge branch 'dz-remove-namespaces-path-uniq' into 'master'

Modify namespace name and path validation

## What does this MR do?

* Allow same namespace name with different parent_id
* Allow same namespace path. Uniq validation should be handled by routes table

## Are there points in the code the reviewer needs to double check?

migrations

## Why was this MR needed?

So we can use same name in different nesting like: 

```
gitlab-ce/foo
gitlab-com/foo
foo/gitlab-ce
```

## Screenshots (if relevant)

no

## Does this MR meet the acceptance criteria?

- ~~[Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added~~
- ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/2772

See merge request !8013

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