fix nightmarish underscore-versus-camelcase confusion in propTypes, and track default URLs for placeholder content
fix nightmarish underscore-versus-camelcase confusion in propTypes, and track default URLs for placeholder content

matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk