Add Contribution Guidelines and Issue / PR Templates

Improve media library performance

The `info` property seems to refer to the image size, not the rotation center in the backdrop library. So if it exists, halve it to find the center.

Towards #18

Remove MediaLibrary helper since it is now unused

Now possible because we send down the correct content type for SVG assets.

add audio engine

Configure port through webpack, not npm

The previous method of configuring the port number was not cross-platform.  Resolves #20

YAGNI: Remove clutter from GUI component

Update in/active states for flag and stop

Pass for overall layout

Make git hooks a bit less cumbersome

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.

`importLoaders` is needed for the way postcss works with css modules.

Update browser target to match scratch-www

It was not being used by GUI so no reason for it to be initialized there

Sprite selector, CSS and Redux overhaul

This will allow the GUIComponent to have control over the layout

The inline css method gives us nothing that CSS modules can't give us. This puts the project more in line with the way www works, and makes CSS actually readable.

Using class names gives us full control of the modal styles.

Refactor SpriteSelector into "Target Pane" which contains the sprite selector, stage selector, and the buttons for new sprites, costumes and backdrops.

Add a new stage selection component that displays the stage separately from the sprite list.

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