Introduce fork_namespace entity and serializer

This serializer and entity will be consumed by fork controller
representing available namespaces for forking via JSON

app/serializers/fork_namespace_entity.rb

+# frozen_string_literal: true
+
+class ForkNamespaceEntity < Grape::Entity
+  include ActionView::Helpers::NumberHelper
+  include RequestAwareEntity
+  include MarkupHelper
+
+  expose :id, :name, :description, :visibility, :full_name,
+         :created_at, :updated_at, :avatar_url
+
+  expose :fork_path do |namespace, options|
+    project_forks_path(options[:project], namespace_key: namespace.id)
+  end
+
+  expose :forked_project_path do |namespace, options|
+    if forked_project = namespace.find_fork_of(options[:project])
+      project_path(forked_project)
+    end
+  end
+
+  expose :permission do |namespace, options|
+    membership(options[:current_user], namespace)&.human_access
+  end
+
+  expose :relative_path do |namespace|
+    polymorphic_path(namespace)
+  end
+
+  expose :markdown_description do |namespace|
+    markdown_description(namespace)
+  end
+
+  expose :can_create_project do |namespace, options|
+    options[:current_user].can?(:create_projects, namespace)
+  end
+
+  private
+
+  # rubocop: disable CodeReuse/ActiveRecord
+  def membership(user, object)
+    return unless user
+
+    @membership ||= user.members.find_by(source: object)
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
+
+  def markdown_description(namespace)
+    markdown_field(namespace, :description)
+  end
+end
+
+ForkNamespaceEntity.prepend_if_ee('EE::ForkNamespaceEntity')

app/serializers/fork_namespace_serializer.rb

+# frozen_string_literal: true
+
+class ForkNamespaceSerializer < BaseSerializer
+  entity ForkNamespaceEntity
+end

ee/app/serializers/ee/fork_namespace_entity.rb

+# frozen_string_literal: true
+
+module EE
+  module ForkNamespaceEntity
+    extend ActiveSupport::Concern
+
+    prepended do
+      expose :marked_for_deletion do |namespace|
+        namespace.marked_for_deletion?
+      end
+    end
+  end
+end

spec/serializers/fork_namespace_entity_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ForkNamespaceEntity do
+  include Gitlab::Routing.url_helpers
+  include ProjectForksHelper
+
+  let_it_be(:user) { create(:user) }
+  let_it_be(:project) { create(:project) }
+
+  let(:namespace) { create(:group, :with_avatar, description: 'test') }
+  let(:entity) { described_class.new(namespace, current_user: user, project: project) }
+
+  subject(:json) { entity.as_json }
+
+  before do
+    project.add_maintainer(user)
+  end
+
+  it 'renders json' do
+    is_expected.not_to be_nil
+  end
+
+  %w[id
+     name
+     description
+     markdown_description
+     visibility
+     full_name
+     created_at
+     updated_at
+     avatar_url].each do |attribute|
+    it "includes #{attribute}" do
+      expect(json[attribute.to_sym]).to be_present
+    end
+  end
+
+  it 'exposes path for forking project to the namespace' do
+    expect(json[:fork_path]).to eq project_forks_path(project, namespace_key: namespace.id)
+  end
+
+  it 'exposes forked_project_path when fork exists in namespace' do
+    namespace.add_maintainer(user)
+    fork_in_namespace = fork_project(project, user, namespace: namespace)
+
+    expect(json[:forked_project_path]).to eql project_path(fork_in_namespace)
+  end
+
+  it 'exposes relative path to the namespace' do
+    expect(json[:relative_path]).to eql polymorphic_path(namespace)
+  end
+
+  it 'sets can_create_project to true when user can create projects in namespace' do
+    allow(user).to receive(:can?).with(:create_projects, namespace).and_return(true)
+
+    expect(json[:can_create_project]).to be true
+  end
+
+  it 'sets can_create_project to false when user is not allowed create projects in namespace' do
+    allow(user).to receive(:can?).with(:create_projects, namespace).and_return(false)
+
+    expect(json[:can_create_project]).to be false
+  end
+end

spec/serializers/fork_namespace_serializer_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ForkNamespaceSerializer do
+  it 'represents WebIdeTerminalEntity entities' do
+    expect(described_class.entity_class).to eq(ForkNamespaceEntity)
+  end
+end