Expand TARA auth flow to registrant portal

app/controllers/registrant/tara_controller.rb

@@ -0,0 +1,33 @@
+class Registrant
+  class TaraController < ApplicationController
+    skip_authorization_check
+
+    # rubocop:disable Style/AndOr
+    def callback
+      session[:omniauth_hash] = user_hash
+      @registrant_user = RegistrantUser.find_or_create_by_omniauth_data(user_hash)
+
+      if @registrant_user
+        flash[:notice] = t(:signed_in_successfully)
+        sign_in_and_redirect(:registrant_user, @registrant_user)
+      else
+        show_error and return
+      end
+    end
+    # rubocop:enable Style/AndOr
+
+    def cancel
+      redirect_to root_path, notice: t(:sign_in_cancelled)
+    end
+
+    def show_error
+      redirect_to new_registrant_user_session_url, alert: t(:no_such_user)
+    end
+
+    private
+
+    def user_hash
+      request.env['omniauth.auth']
+    end
+  end
+end