Allow intl to be passed in to GUI as a prop (rather than having to be already in the store)

* Added object.assign polyfill

* Fix IE load failure

Don’t call `(window.AudioContext || window.webkitAudioContext)()` if platform is IE

Catch any other errors, and display Browser unsupported in error-boundary.

If for some reason there are no other errors (unlikely), preview-modal will still check for IE and show the unsupported modal.

Polyfilling Object.assign is enough to get through load and show the modal for IE. We can always switch to `babel-polyfill` if there are others needed later, but for now this is smaller.

Added language-selector component and container. Replaced lib/intl.jsx with reducers/intl.js and added updateIntl to handle language changes.

Moved languages.json into src

Using react-intl-redux package.

This just sets up the default English. Language switching in another PR. Requires locale/messages.json to exist, `{}` is ok.

Costumes and Sound tab strings.

It wasn't ever necessary. Still allow configuring this path in the GUI in case of different paths to static files for scratch-blocks.

Otherwise the VM can't JSON.load it.

If the app is loading with an initial project id, don't try to load the default project. Previously, this data would load on top of the project being loaded.

Does not fix the issue of blocks not loading in the blocks pane, which appears to be separate.

With jscodeshift thank God

These aren't necessary to update in realtime, so throttle them to once every 30ms

Move layout styles from component stylesheets to the GUI, or the component that is placing the component.

Add a new `Box` component to control layouts, basically giving a convenient way to lay out with flexbox.

Use `Box` as the output component for any component that may need to be passed layout options. The props passing convention allows giving layout props to the final `Box` component, making wrapping with another `Box` unnecessary in some cases.

In preparation to split the sprites from the stage, start managing the target list state in a single place.

Move behavior-related components to "containers", leave presentation components in "components".

While doing so:
- Use ProjectLoader for loading projects
- Stop using loadEmptyProject and supply empty project data
- Move project loading to App, GUI now receives data (as a string - the vm should receive an object eventually)
- Add logging

This way we can have a consistent style for Node and web code.

Thanks to babel-loader and babel-eslint, we can use the same linting everywhere.  This makes our imports easier since we don't need to think about the extension.

Also export things more succinctly in src/index.js