animated droppable placeholder spike

[jaredcrowe]

A spike of an animated droppable placeholder. My thoughts:

• It might be better to make placeholder a connected component rather than passing down the phase via props.
• The way I've done dynamic CSS animations feels a bit 😕. I'm not sure if CSS transitions will work cleanly but it might be worth exploring. Otherwise we might have to do animations with JS.
• There's probably a layer of memoization and optimisation which could sit on top of some of the functions in here.

Anyway, proof of concept 🤗

[alexreardon]

As discussed - we will not be tackling this as part of #2 in order to ship the feature a bit quicker. We can add this in afterwards 👍

Issue to track this work: #77

[alexreardon]

For future purposes: I think it will be cleaner to avoid needing to manage a stylesheet for the animation. We can just use transition and some componentDidMount trickery. Similar to what we do with Reveal in @atlaskit/navigation