Merge branch 'environment-entity-inherit' into 'master'

entities: make Environment inherit EnvironmentBasic

## What does this MR do?

Makes the `Environment` entity inherit from `EnvironmentBasic`

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

No.

## Why was this MR needed?

Just a code simplification.

## What are the relevant issue numbers?

N/A

## Screenshots (if relevant)

N/A

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] All builds are passing
- [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 you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5959

lib/api/entities.rb

@@ -514,13 +514,12 @@ module API
       expose :duration
     end
 
-    class Environment < Grape::Entity
+    class EnvironmentBasic < Grape::Entity
       expose :id, :name, :external_url
-      expose :project, using: Entities::Project
     end
 
-    class EnvironmentBasic < Grape::Entity
-      expose :id, :name, :external_url
+    class Environment < EnvironmentBasic
+      expose :project, using: Entities::Project
     end
 
     class Deployment < Grape::Entity