Migrate to WebExtensions

[strugee]

This addon will become unsupported with Firefox 57. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Porting_a_legacy_Firefox_add-on for migration docs.

[strugee]

I actually wonder if it makes sense to just deprecate this project and instead port upstream. https://www.extensiontest.com/ has tests for how easy this would be.

[strugee]

Just ran a test and it looks like upstream is theoretically 100% compatible. Therefore filed panicsteve/cloud-to-butt#65 suggesting merging the two projects.

[Hellenab]

Ah, so it's already ported https://addons.mozilla.org/en-US/firefox/addon/cloud-2-butt-plus/
Just pasting from the link :)