Our CDN-rooted HTTPS URLs are incompatible with IE-on-XP

[reefdog]

This is just a recording ticket for posterity.

We recently began serving most platform and embed static assets from a CDN, rooted at assets.documentcloud.org. For HTTPS, this required the use of SNI. SNI is notably not supported by IE on Windows XP, which means IE-on-XP visitors to the HTTPS version of our site, or who load our embeds (which are protocol-agnostic) from an HTTPS parent page, get no content. (On the upcoming page embeds, they'll at least get the unenhanced fallback view.)

With both https://analytics.usa.gov/ and our own analytics showing XP usage at ~2.5% and IE-on-XP specifically at 0.35% (for the platform only, not including embeds), and with nobody yet complaining since we enabled the CDN, I think we can safely ignore this.