Uploader as workaround for browser-only users

CORS errors prevent browser-only users from really using blanket as a drop-in coverage tool.  To offer some functionality to them I've created a warning dialog with an uploader feature.  Select the directory (or files,in FF) where the source files are located.  The page will refresh and re-try the instrumentation without requirejs (and hence, without CORS issues).

This feature is pretty experimental, and is just designed to encourage users to use blanket and learn more about testing.  Hopefully it will encourage them to use a local server, or learn about headless testing (where the CORS issues aren't a problem).