rename container class to excalidraw and move css from index.html to app.css (#1729)

Moved the css from index.html to app.css so it can be included in upstream app as well

src/components/App.tsx

@@ -209,7 +209,7 @@ class App extends React.Component<any, AppState> {
     const canvasHeight = canvasDOMHeight * canvasScale;
 
     return (
-      <div className="container">
+      <div className="excalidraw">
         <LayerUI
           canvas={this.canvas}
           appState={this.state}

src/css/styles.scss

@@ -51,7 +51,7 @@ canvas {
   image-rendering: -moz-crisp-edges; // FF
 }
 
-.container {
+.excalidraw {
   display: flex;
   position: fixed;
   top: 0;