Instead of creating a new mechanism for the client's deliverer to set
the sidebar app URL and asset root, re-use the existing documented
system for configuring the client.

 - Rename 'appHtmlUrl' to 'sidebarAppUrl' to be more obvious to new
   developers.
 - Make `settings.js` usable without a polyfill for Object.assign()
   so that it can be used in the very small boot script.