Allow serving web interface from a subdirectory #22

[mremallin]

This PR is to enable support for the following deployment model:

Today: https://photos.example.ca
PR: https://example.ca/photos

Summary of changes:

• Added a new config variable at NPM build time to track the deployment folder: webroot_base_folder
  • Default is "/" to support the current deployment model
  • Supporting a sub-folder must specify the full path from root. Eg. "/photos/"
• Update the NPM build to apply the above variable across the generated code/css/html
• Added 001-config.js for a single place to store the generated config variable
• Updated many link generators in the code to use the new get_webroot_folder() for paths
• Moved index.html and loading.gif to be generated as part of the NPM build

Manual Testing:

• Running serve.py for both "/" and "/photos/"
• 1 level of subfolder
• Forward and back in/out of fullscreen
• Pasting the link from address bar into a new tab to load an image directly

EDIT: Updated URLs to example