Add HTTPS Switch Planner mode to devtools

Yan, I wound up using your idea to put this in devtools. It definitely makes a lot more sense, thanks for the suggestion. And the panel lifecycle provides an easy way to avoid slowing down the whole browser and storing unwanted history information.

Readymade crx for testing: https://jacob.hoffman-andrews.com/hacks/https-everywhere-jsha-switch-planner-demo-v2.crx (and .asc for sig).