Refactor CSS and React logic for layout (#1424)

* Refactor CSS layout and react logic for layout

Fixes https://github.com/chanzuckerberg/cellxgene/issues/1022

* Menubar should wrap inside middle pane instead of overlapping left
  sidebar when window is scrunched
* cellxgene should have a minimum width of 1240px

1. Replace absolute positioning and dimension calculation with css grid
2. Use flexbox for wrapping menubar buttons

* Middle pane (graph) can calculate its own size
* Removing components calculating their size/position relative to
  eachother increases modularity, decreases use of global variables
* Improved some scrollbar behavior
* Removed responsive reducer, propagating window size to components
  triggers unnecessary events and encourages breaking modularity; doing
  this made some components state agnostic

Reference: https://css-tricks.com/snippets/css/complete-guide-grid/

* Reposition the continuous legend

* Small fixes

* Respond to feedback from @colinmegill

* Respond to feedback from @colinmegill

Add more documentation on the renderGraph method.

client/src/components/graph/overlays/centroidLabels.js

@@ -13,7 +13,8 @@ export default
 }))
 class CentroidLabels extends PureComponent {
   // Check to see if centroids have either just been displayed or removed from the overlay
-  componentDidUpdate = (prevProps) => {
+
+  componentDidUpdate(prevProps) {
     const { labels, overlayToggled } = this.props;
     const prevSize = prevProps.labels.size;
     const { size } = labels;