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/components/menubar/infoMenu.js

@@ -3,12 +3,23 @@ import React from "react";
 import { Button, Popover, Menu, MenuItem, Position } from "@blueprintjs/core";
 
 function InformationMenu(props) {
-  const { libraryVersions } = props;
+  const { libraryVersions, aboutLink } = props;
   return (
     <div style={{ marginLeft: 10 }} className="bp3-button-group">
       <Popover
         content={
           <Menu>
+            {aboutLink ? (
+              <MenuItem
+                href={aboutLink}
+                target="_blank"
+                icon="document-open"
+                text="About this dataset"
+              />
+            ) : (
+              ""
+            )}
+
             <MenuItem
               href="https://chanzuckerberg.github.io/cellxgene/faq.html"
               target="_blank"