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/autosave/index.js

@@ -80,10 +80,11 @@ class Autosave extends React.Component {
             : "autosave-complete"
         }
         style={{
-          position: "fixed",
+          position: "absolute",
           display: "inherit",
-          right: globals.leftSidebarWidth + 5,
-          bottom: 5,
+          right: 8,
+          bottom: 8,
+          zIndex: 1,
         }}
       >
         {this.statusMessage()}