Improve browser bundling with Browserify/Webpack

This adds a `"browser"` field to the `package.json` file to hint to both Browserify and Webpack that they should _not_ include the data for all locales (just English) when bundling for the browser.

`"browser"` spec: https://gist.github.com/defunctzombie/4339901#ignore-a-module
We can do a pass over our libs to explicitly add `.js` to our internal imports — but in another PR, since that's out of scope for this set of changes.