index.html: single CDN, SRI, update deps
index.html: single CDN, SRI, update deps

Updates jquery to 3.4.1
Updates stackdriver-errors-js to 0.7.0
Updates toastr to 2.4.1
Adds subresource integrity to validate remote JS
Using a single CDN because rawgit is EOL.
A single CDN will also reduce round trip times.
unpkg is well used and is provided via cloudflare.

ubershmekel/redditp,ubershmekel/redditp