matrix-js-sdk now retains redacted events. Filter them out of the timeline.
Also put empty placeholders in the dom so that if we try to scroll to a
redacted event, we don't end up blowing up.
This adds support for links to particular event ids: add /<eventId> to the URL
for a room.
This commit also ensures that we scroll to the 'read marker' when switching to
a room which has no previous scroll state, as well as preventing that marker
from going past the middle of the screen.
This also reinstates the preservation of scroll state when switching rooms,
which was disabled previously.
