Control currently viewied event via RoomViewStore

Fix for https://github.com/vector-im/riot-web/issues/4224

Due to the way `MatrixChat` does a state update when the `view_room` dispatch fires and a second update when `RoomViewStore` sends an update, the current event ID and room ID were becoming out of sync. The solution devised was to have the event ID managed by the `RoomViewStore` itself and do any defaulting there (for when we revisit a room that we saved scroll state for previously).

This required a few changes:
 - The addition of `update_scroll_state` in `RoomViewStore` allows the `RoomView` to save scroll state for a room before swapping to another one. Previously the caching of scroll state was done in `RoomView`.
 - The `view_room` dispatch now accepts an `event_id`, which dictates which event is supposed to be scrolled to in the `MessagePanel` when a new room is viewed. It also accepts `event_offset`, but currently, this isn't passed in by a dispatch in the app, but it is clobbered when loading the default position when an `event_id` isn't specified. Finally, `highlighted` was added to distinguish whether the initial event being scrolled to is also highlighted. This flag is also used by `viewRoom` in `MatrixChat` in order to decide whether to `notifyNewScreen` with the specified `event_id`.

src/components/structures/LoggedInView.js

@@ -223,10 +223,8 @@ export default React.createClass({
                         ref='roomView'
                         autoJoin={this.props.autoJoin}
                         onRegistered={this.props.onRegistered}
-                        eventId={this.props.initialEventId}
                         thirdPartyInvite={this.props.thirdPartyInvite}
                         oobData={this.props.roomOobData}
-                        highlightedEventId={this.props.highlightedEventId}
                         eventPixelOffset={this.props.initialEventPixelOffset}
                         key={this.props.currentRoomId || 'roomview'}
                         opacity={this.props.middleOpacity}