Added the very useful .animation-fill-mode() mixin

I think that's the only animation property that was missing and it's quite useful.

https://developer.mozilla.org/en-US/docs/Web/CSS/animation-fill-mode
Add sitemap.xml and robots.txt files.
Fixes #12851: it's not a tumah
Merge pull request #12866 from twbs/sitemap

Add sitemap.xml and robots.txt files.
Merge pull request #12855 from lipis/patch-1

Added the very useful .animation-fill-mode() mixin
100% less tumah