Add import/namespaces controller

This MR adds a new, tiny `import/namespaces` controller.

This controller lists available namespaces for import. Moving this to
separate controller has following rationale:

* `namespaces` are required for each controller, provided in `import/`
  folder. They are identical, does not have any customization, etc.

* ATM `namespaces` are returned as the part of `status` in
  `base_controller`. With our intention to add pagination to `status` we
  need to move "static part of the content" (namespaces) away from part,
  which will be paginated (repos)

Related issues:

* https://gitlab.com/gitlab-org/gitlab/-/issues/233059
* https://gitlab.com/gitlab-org/gitlab/-/issues/222889
* https://gitlab.com/gitlab-org/gitlab/-/issues/27370

app/controllers/import/available_namespaces_controller.rb

+# frozen_string_literal: true
+
+class Import::AvailableNamespacesController < ApplicationController
+  def index
+    render json: NamespaceSerializer.new.represent(current_user.manageable_groups_with_routes)
+  end
+end

config/routes/import.rb

@@ -8,6 +8,8 @@ Devise.omniauth_providers.map(&:downcase).each do |provider|
 end
 
 namespace :import do
+  resources :available_namespaces, only: [:index], controller: :available_namespaces
+
   resource :github, only: [:create, :new], controller: :github do
     post :personal_access_token
     get :status

spec/controllers/import/available_namespaces_controller_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Import::AvailableNamespacesController do
+  let_it_be(:user) { create(:user) }
+  let_it_be(:manageable_groups) { [create(:group), create(:group)] }
+
+  before do
+    sign_in(user)
+    manageable_groups.each { |group| group.add_maintainer(user) }
+  end
+
+  describe "GET index" do
+    it "returns list of available namespaces" do
+      unrelated_group = create(:group)
+
+      get :index
+
+      expect(response).to have_gitlab_http_status(:ok)
+      expect(json_response).to be_kind_of(Array)
+
+      response_ids = json_response.map { |n| n["id"] }
+
+      expect(response_ids).not_to include(unrelated_group.id)
+      expect(response_ids).to contain_exactly(*manageable_groups.map(&:id))
+    end
+
+    context "with an anonymous user" do
+      before do
+        sign_out(user)
+      end
+
+      it "redirects to sign-in page" do
+        get :index
+
+        expect(response).to redirect_to(new_user_session_path)
+      end
+    end
+  end
+end