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/components/categorical/category/index.js

@@ -205,7 +205,7 @@ class Category extends React.Component {
       return (
         <div style={{ marginBottom: 10, marginTop: 4 }}>
           <span style={{ fontWeight: 700 }}>
-            {truncatedString ? truncatedString : metadataField}
+            {truncatedString || metadataField}
           </span>
           : {schema.annotations.obsByName[metadataField].categories[0]}
         </div>