Make ReCaptcha's "I'm not a robot" accurate

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.