* 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.