Swap out rawgit for jsDelivr for the inert polyfill

[ericwbailey]

RawGit has been placed in sunset mode, with a disclosure that support may end in October 2019.

jsDelivr offers a RawGit migration, so here's Google's inert polyfill re-hosted:
https://cdn.jsdelivr.net/gh/GoogleChrome/inert-polyfill@v0.1.0/inert-polyfill.min.js

[scottaohara]

To close this issue, all the HTML files that link to the rawgit hosted file need to be changed to the link Eric posted.