Merge branch '18583-implement-access-request-to-project-group-api' into 'master'

Add group members API endpoints to request access and approve requests

## What does this MR do?

Add group members API endpoints to request access and approve requests.

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

I chose to factorize the group/project members API as well as the new group/project access requests API.

I initially also created new services to centralize the permission checks related to actions on members, but this was out of scope and this MR is already quite big so I opened a temp MR at !5566 based on this one.

## Why was this MR needed?

To finish the "request access" feature.

## What are the relevant issue numbers?

Closes #18583.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [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 !4833

3 files changed, 8 insertions, 2 deletions

diff --git a/app/models/members/project_member.rb b/app/models/members/project_member.rb
index f176feddbad..18e97c969d7 100644
--- a/app/models/members/project_member.rb
+++ b/app/models/members/project_member.rb
@@ -8,6 +8,7 @@ class ProjectMember < Member
   # Make sure project member points only to project as it source
   default_value_for :source_type, SOURCE_TYPE
   validates_format_of :source_type, with: /\AProject\z/
+  validates :access_level, inclusion: { in: Gitlab::Access.values }
   default_scope { where(source_type: SOURCE_TYPE) }
 
   scope :in_project, ->(project) { where(source_id: project.id) }
diff --git a/app/models/project.rb b/app/models/project.rb
index 3b1a53edc75..e0b28160937 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -999,6 +999,10 @@ class Project < ActiveRecord::Base
     project_members.find_by(user_id: user)
   end
 
+  def add_user(user, access_level, current_user = nil)
+    team.add_user(user, access_level, current_user)
+  end
+
   def default_branch
     @default_branch ||= repository.root_ref if repository.exists?
   end
diff --git a/app/services/members/destroy_service.rb b/app/services/members/destroy_service.rb
index 15358f80208..9e3f6af628d 100644
--- a/app/services/members/destroy_service.rb
+++ b/app/services/members/destroy_service.rb
@@ -2,8 +2,9 @@ module Members
   class DestroyService < BaseService
     attr_accessor :member, :current_user
 
-    def initialize(member, user)
-      @member, @current_user = member, user
+    def initialize(member, current_user)
+      @member = member
+      @current_user = current_user
     end
 
     def execute