Merge branch 'dz-nested-groups' into 'master'

Add nested groups support on data level

## What does this MR do?

- [x] Add  `parent_id` field to `Namespace`model. 
- [x] Create new database table `routes` that keeps information about full path to each group or project
- [x] Remove uniq index from `namespaces.path`
- [x] Add uniq index on `routes.path`
- [x] Fill routes table with path data from namespaces and projects
- [x] Change Namespace/Project URL lookup by routes table
- [x] Rename related routes (nested groups, projects) when parent path changes

This is solely backend preparation. UI, Permissions and API support will be added in separate merge request.   
 
## Are there points in the code the reviewer needs to double check?

migrations, Route model, Routable concern

Will require downtime. See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7121#note_19490281 discussion

## Why was this MR needed?

One step further to full nested groups support

## Screenshots (if relevant)

No UI changes in this merge request so far

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry 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 !7121

1 files changed, 8 insertions, 0 deletions

diff --git a/spec/models/namespace_spec.rb b/spec/models/namespace_spec.rb
index ba0ed4a3603..7f82e85563b 100644
--- a/spec/models/namespace_spec.rb
+++ b/spec/models/namespace_spec.rb
@@ -124,4 +124,12 @@ describe Namespace, models: true do
       expect(Namespace.clean_path("--%+--valid_*&%name=.git.%.atom.atom.@email.com")).to eq("valid_name")
     end
   end
+
+  describe '#full_path' do
+    let(:group) { create(:group) }
+    let(:nested_group) { create(:group, parent: group) }
+
+    it { expect(group.full_path).to eq(group.path) }
+    it { expect(nested_group.full_path).to eq("#{group.path}/#{nested_group.path}") }
+  end
 end