add live stream viewing to React prototype

It's a start. It can display several streams at once, which is nice.
There are lots of opportunities for improvement:

*   it doesn't keep the videos approximately in sync.
*   it accumulates extra buffering, drifting behind live. This is
    particularly noticeable when it's paused and played again; it can
    be several seconds before it jumps to after the break.
*   it always uses the sub stream rather main. I'd prefer it support
    "auto" (use main if the viewport is larger than the sub stream and
    there's sufficient bandwidth), "main", or "sub".
*   it has a kludgy heuristic where it throws away everything buffered 5
    seconds before the current timestamp. It should throw away
    everything before the current GOP instead, but I need to alter the
    API so it can easily know when that is.
*   it can't tell you when a camera connection is down. This needs an
    API change also.
*   it'd be nice to quickly double-click on a stream to view only it,
    then double-click again to go back to the multi-pane view.
*   it doesn't allow you to zoom in on part of the video. This would be
    nice particularly when viewing 4k video streams on small screens.
*   it has only four preconfigured layouts that subdivide a 16x9
    viewport. You have to choose every camera every time. It'd be nice
    to both allow more flexibility and have more memory.

React prototype: #111
live stream: #59

ui/src/List/index.tsx

@@ -73,10 +73,10 @@ const useStyles = makeStyles((theme: Theme) => ({
 interface Props {
   timeZoneName: string;
   cameras: Camera[];
-  showMenu: boolean;
+  showSelectors: boolean;
 }
 
-const Main = ({ cameras, timeZoneName, showMenu }: Props) => {
+const Main = ({ cameras, timeZoneName, showSelectors }: Props) => {
   const classes = useStyles();
 
   /**
@@ -134,7 +134,7 @@ const Main = ({ cameras, timeZoneName, showMenu }: Props) => {
     <div className={classes.root}>
       <Box
         className={classes.selectors}
-        sx={{ display: showMenu ? "block" : "none" }}
+        sx={{ display: showSelectors ? "block" : "none" }}
       >
         <StreamMultiSelector
           cameras={cameras}