Fix the URLs to the GitHub emoji CDN.

Review Request #1653 — Created May 13, 2019 and updated

Review Board
GitHub changed the CDN used to host their assets, like emoji. The
pymdown-extensions library we use still refers to the old URL, meaning
that all emoji images we try to load fail. This change overrides the
URLs, pointing to the modern CDN.

Testing Done:
Tested Unicode-based Emoji URLs and non-standard Emoji URLs. Verified
both kinds could be loaded from the GitHub CDN.

Bugs closed: 4817

Reviewed at