CORE-256 Support multiple auditors in UI

This PR adds support for adding and removing arbitrary numbers of auditors to/from an audit, both immediately in the tree view and deferredly in the modal while creating or editing an Audit.  This does not require changes to the data layer because Auditors have always been set through UserRole authorizations (which naturally have 1-to-n mapping to Audit contexts).  This involved the creation of a component for adding arbitrary connected objects to an object being edited in the modal, which is called <ggrc-modal-connector>, as well as amending <ggrc-quick-add> to support the adding of authorizations to the context.  Also the UserRole object can now be created with role_name as a string instead of role as an object, and it will look up the appropriate role by name before saving.