Add configurable about this dataset links (#907)

* add about arg

* add simple url validator

* attach about link to config api

* add links to configDefaults

* add conditional link in top left and menu item

* whitespace

* change to lower case

* move --about arg before click.command()

if this fixes it I have no idea why

* change link>URL

* be more descriptive about URL

* Make error more explicit

* refactor attach_data to accept about

* format

* change icon

* add trailing parenthesis

* whitespace

client/src/globals.js

@@ -15,7 +15,8 @@ export const configDefaults = {
   displayNames: {},
   parameters: {
     "max-category-items": 1000
-  }
+  },
+  links: {}
 };
 
 /* colors */