Fix the URLs to the GitHub emoji CDN.
Review Request #10543 — Created April 26, 2019 and submitted
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.
Tested Unicode-based Emoji URLs and non-standard Emoji URLs. Verified
both kinds could be loaded from the GitHub CDN.
Bug 4817 was just filed