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/subset.js

@@ -1,5 +1,6 @@
 import React from "react";
 import { AnchorButton, ButtonGroup, Tooltip } from "@blueprintjs/core";
+import styles from "./menubar.css";
 import * as globals from "../../globals";
 
 function Subset(props) {
@@ -11,18 +12,21 @@ function Subset(props) {
   } = props;
 
   return (
-    <ButtonGroup style={{ marginRight: "10px" }}>
+    <ButtonGroup
+     className={styles.menubarButton}
+    >
       <Tooltip
         content="Subset to currently selected cells and associated metadata"
         position="bottom"
         hoverOpenDelay={globals.tooltipHoverOpenDelay}
       >
         <AnchorButton
+          type="button"
           data-testid="subset-button"
           disabled={!subsetPossible}
           icon="pie-chart"
           onClick={handleSubset}
-        />
+  />
       </Tooltip>
       <Tooltip
         content="Undo subset and show all cells and associated metadata"
@@ -30,6 +34,7 @@ function Subset(props) {
         hoverOpenDelay={globals.tooltipHoverOpenDelay}
       >
         <AnchorButton
+          type="button"
           data-testid="reset-subset-button"
           disabled={!subsetResetPossible}
           icon="full-circle"