Show target when dragging cards

[chrislaskey]

Adds a target area when dragging cards between lists. Implemented using CSS styles to display the card's target location while dragging based on the monitor.isOver() functionality in React DnD.

A CSS based solution like this can't handle all the edge cases, like dragging cards to an empty list.

But it does handle the primary one of dragging cards around within a list and between lists very well. It works well with the CSS transition and creates a nice overall effect.

This revealed two existing drag and drop behaviors which are unrelated to these changes:

1. Picking up a card and dropping back in the same spot will cause the card to swap places with the one below it. In all other cases the behavior is for the current card to go above the card its hovering over.
2. There's some consistent failures that occur when trying to update the card order, which you can see in the console when dragging around cards.

This PR doesn't attempt to tackle either of these issues.

Hope this code helps, this is a great learning project and really shows off the power of both Phoenix + Elixir and React + Redux!

[bigardone]

Hi @chrislaskey! Thank you very much for this PR, this was on top of my to-do list since the beginning, but I never had time to solve it. I love the result, great job!
Just a minor comment for the sake of consistency, do you mind renaming the isOver class name to is-over? I'm following the convention of naming classes using-dashes and id's using_underscores... I hope you don't mind!
Thank you very much again :)