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/brushableHistogram/index.js

@@ -88,8 +88,8 @@ class HistogramBrush extends React.PureComponent {
   constructor(props) {
     super(props);
 
-    this.marginLeft = 12; // Space for 0 tick label on X axis
-    this.marginRight = 52; // space for Y axis & labels
+    this.marginLeft = 10; // Space for 0 tick label on X axis
+    this.marginRight = 54; // space for Y axis & labels
     this.marginBottom = 25; // space for X axis & labels
     this.marginTop = 3;