Add support for touch devices (mobile, tablet, etc)

Avion's user story map currently only works on desktop machines, with support for a mouse / touchpad. We could extend this so that you can interact with the user story map via a touch device, with support for drag and drop, etc.

This would be useful when product managers are on the go but have a new idea and want to jot something down, write a new story, etc. Additionally, many tactile devices (such as the Apple iPad Pro), are now being used as a decent, long-term option for day-to-day work in the office.