Make ReCaptcha text accurate per Strugee

Correct the "I'm not a robot" text of ReCaptcha to be more accurate - the new-and-improved version says "I want to do unpaid image classification". ReCaptcha sucks for lots of reasons - it's annoying, arbitrary, and extremely hostile to Tor users - but this is the only problem fixable in a WebExtension.

The extension icon was stolen from Google's Material Design icon set because the Mozilla WebExtension documentation said their sample was from there and then I decided that I enjoyed irony. The icon is licensed Apache 2.0. If someone designs an actual icon I'll probably replace the current one with it.

See the blog post for the background and inspiration for this extension.