Sync with original sage theme

[robbinjohansson]

This repo needs to be synced with the original fork (https://github.com/roots/sage) as it's heavily outdated, last updated May 14, 2018.

One way to do this is to add the original repo as a new upstream remote, fetch and merge the commits, resolve the conflicts and make sure everything is working as expected :)

Useful links on adding a git remote, and merging with unrelated histories:
https://medium.com/@_oleksii_/how-to-synchronize-two-remote-git-repositories-e63b78892901
https://gist.github.com/vinothkannans/323898332ab571d80b1eeb6a037fa2ad