Add 'startAtBottom' flag
Add 'startAtBottom' flag

Controls whether a scrollPanel starts off at the bottom.

This may not be necessary and could either be derived from stickyBottom, but
this means I can be sure that the behaviour of ScrollPanel is completely
unchanged for all other uses to avoid breaking any other uses of
ScrollPanel.

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