Add user-defined category-label colors (#1402)

* Add user-defined category-label colors

Fixes https://github.com/chanzuckerberg/cellxgene/issues/1152

As described in https://github.com/chanzuckerberg/cellxgene/issues/1307

* Respond to feedback from @bkmartinjr in nodejs

* Respond to feedback from @bkmartinjr in python

* Add tests to the server module

* Autoformat python, run linter

* Make colors_get error handling specific

* Respond to feedback from @bkmartinjr

* Respond to feedback from @bkmartinjr

* Fix whitespace

* Fix python lint errrors

* Update documentation

* Add --disable-user-colors option to launch and cxgtool.py

* Fix python formatting

* Rename '--disable-user-colors' to '--disable-custom-colors'

client/src/reducers/undoableConfig.js

@@ -12,6 +12,7 @@ const skipOnActions = new Set([
   "interface reset started",
   "initial data load start",
   "universe: column load success",
+  "universe: user color load success",
   "universe exists, but loading is still in progress",
   "configuration load complete",
   "increment graph render counter",