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'

server/test/test_datasets/fixtures.py

@@ -0,0 +1,12 @@
+pbmc3k_colors = {
+    "louvain": {
+        "B cells": "#2ca02c",
+        "CD14+ Monocytes": "#ff7f0e",
+        "CD4 T cells": "#1f77b4",
+        "CD8 T cells": "#d62728",
+        "Dendritic cells": "#e377c2",
+        "FCGR3A+ Monocytes": "#8c564b",
+        "Megakaryocytes": "#bcbd22",
+        "NK cells": "#9467bd",
+    }
+}