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/menubar/clip.js

@@ -9,6 +9,7 @@ import {
   Tooltip,
 } from "@blueprintjs/core";
 import { tooltipHoverOpenDelay } from "../../globals";
+import styles from "./menubar.css";
 
 function Clip(props) {
   const {
@@ -35,10 +36,7 @@ function Clip(props) {
 
   return (
     <div
-      className="bp3-button-group"
-      style={{
-        marginRight: 10,
-      }}
+      className={`bp3-button-group ${styles.menubarButton}`}
     >
       <Popover
         target={