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

2 files changed, 2 insertions, 2 deletions

diff --git a/lib/api/helpers.rb b/lib/api/helpers.rb
index 8db2678b368..8b0f8deadfa 100644
--- a/lib/api/helpers.rb
+++ b/lib/api/helpers.rb
@@ -111,7 +111,7 @@ module API
       if id =~ /^\d+$/
         Group.find_by(id: id)
       else
-        Group.find_by(path: id)
+        Group.find_by_full_path(id)
       end
     end
 
diff --git a/lib/constraints/group_url_constrainer.rb b/lib/constraints/group_url_constrainer.rb
index 5711d96a586..bae4db1ca4d 100644
--- a/lib/constraints/group_url_constrainer.rb
+++ b/lib/constraints/group_url_constrainer.rb
@@ -4,7 +4,7 @@ class GroupUrlConstrainer
 
     return false unless valid?(id)
 
-    Group.find_by(path: id).present?
+    Group.find_by_full_path(id).present?
   end
 
   private