Content scripts should match about:blank
Content scripts should match about:blank

As mentioned in https://bugzilla.mozilla.org/show_bug.cgi?id=1475831 it
should protect data URL pages (see #208) - but it doesn't...

Reopens #165

kkapsner/CanvasBlocker,kkapsner/CanvasBlocker,kkapsner/CanvasBlocker