Refactor layout (#486)

* Refactor to move `<DatasetContainer>` out of layout
* `<DatasetContainer>` is now in the methodology page. Starting the review there will help understand the motive for this change.
* Fixed 404 page which seems way out of date.
* Use row/col grid syntax

client/src/components/J40MainGridContainer.tsx

@@ -0,0 +1,30 @@
+// Trussworks GridContainer won't allow it to span 100% of the page, so
+// this works around it and tries to hide the complexity in component
+import React, {ReactNode} from 'react';
+import {GridContainer} from '@trussworks/react-uswds';
+
+interface ILayoutProps {
+  children: ReactNode,
+  fullWidth?: boolean,
+  className?: string
+}
+
+const J40MainGridContainer = ({
+  children,
+  fullWidth = false,
+  className = 'j40-grid-container'}: ILayoutProps) => {
+  return fullWidth ? (
+    <div
+      className={'j40-grid-container ' + className}>
+      {children}
+    </div>
+  ) : (
+    <GridContainer
+      containerSize={'desktop-lg'}
+      className={'j40-grid-container ' + className}>
+      {children}
+    </GridContainer>
+  );
+};
+
+export default J40MainGridContainer;