Disable Github Importer API by settings

The Github importer can be disable in the settings. the API must deny
the requests when this setting is disabled.

changelogs/unreleased/security-disable-github-import-api-by-seetings.yml

+---
+title: Disable Github Importer API by settings
+merge_request:
+author:
+type: security

lib/api/import_github.rb

@@ -4,6 +4,10 @@ module API
   class ImportGithub < Grape::API
     rescue_from Octokit::Unauthorized, with: :provider_unauthorized
 
+    before do
+      forbidden! unless Gitlab::CurrentSettings.import_sources&.include?('github')
+    end
+
     helpers do
       def client
         @client ||= Gitlab::LegacyGithubImport::Client.new(params[:personal_access_token], client_options)

spec/requests/api/import_github_spec.rb

@@ -26,6 +26,18 @@ describe API::ImportGithub do
       end
     end
 
+    it 'rejects requests when Github Importer is disabled' do
+      stub_application_setting(import_sources: nil)
+
+      post api("/import/github", user), params: {
+        target_namespace: user.namespace_path,
+        personal_access_token: token,
+        repo_id: non_existing_record_id
+      }
+
+      expect(response).to have_gitlab_http_status(:forbidden)
+    end
+
     it 'returns 201 response when the project is imported successfully' do
       allow(Gitlab::LegacyGithubImport::ProjectCreator)
         .to receive(:new).with(provider_repo, provider_repo.name, user.namespace, user, access_params, type: provider)