cleaning up code according to coding standards

js/members.ref

js/private_message_members_widget.js

js/private_message_thread.js

private_message.module

src/Controller/AjaxController.php

src/Form/PrivateMessageForm.php

src/Mapper/PrivateMessageMapper.php

...ld/FieldFormatter/PrivateMessageThreadMemberFormatter.php

...d/FieldFormatter/PrivateMessageThreadMessageFormatter.php

src/Service/PrivateMessageService.php