Changes include:
- Use `bin-loader` instead of `arraybuffer-loader` since the latter is
only compatible with browsers, not with Node itself.
- Use `got` instead of `xhr` since `xhr` only works in browsers and
`got` is nicer anyway.
- Add `json-loader` to Webpack config since `got` requires it.
- Use lower-case values in `DataFormat` to match canonical file names.
- Update `ScratchStorage.addWebSource` to match `WebHelper`'s new API.
- Fix an error in the web helper which was causing an infinite loop on
error.
