Allow users to specify label->color mapping (#3879)

Users can define `label_colors` in a dashboard's JSON metadata that
enforces a label to color mapping.

This also makes the function that maps labels to colors case insensitive.

docs/faq.rst

@@ -221,3 +221,19 @@ When adding columns to a table, you can have Superset detect and merge the
 new columns in by using the "Refresh Metadata" action in the
 ``Source -> Tables`` page. Simply check the box next to the tables
 you want the schema refreshed, and click ``Actions -> Refresh Metadata``.
+
+Is there a way to force the use specific colors?
+------------------------------------------------
+
+It is possible on a per-dashboard basis by providing a mapping of
+labels to colors in the ``JSON Metadata`` attribute using the
+``label_colors`` key.
+
+..code::
+
+	{
+	  "label_colors": {
+		"Girls": "#FF69B4",
+		"Boys": "#ADD8E6"
+	  }
+	}