Add 'white' as a backgronud color for paragraphs

We'll use in the title paragraph to ensure menu can always be seen (does *not* get
'transparent' in this case).

Ref agaric/sites/agaric-com#86

config/sync/field.storage.paragraph.field_style_color.yml

@@ -32,6 +32,9 @@ settings:
     -
       value: grey
       label: Gray
+    -
+      value: white
+      label: White
   allowed_values_function: ''
 module: options
 locked: false