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.
Given we want to use isAtBottom to figure out whether to show 'unread messages'
counts, we ought to return the current scroll state, rather than the saved one.
This fixesvector-im/vector-web#576
The dance to avoid doing repeated fill requests on every update is common, so
add it to ScrollPanel. Let onFillRequest return a promise, which prevents any
updates until it completes.
