Support drag/drop on mobile devices

[richardtallent]

The drag and drop functionality currently does not support touch devices. I haven't looked into whether this is easy enough to do in vanilla JS or if adding a dependency to deal with various platform differences will be more practical.

May want to support "ghost" positioning during the drag event (i.e., being able to see the other events "open up" the place where the current event would be positioned).

[richardtallent]

Moved milestone to v5. If we ever do this, we may also need to make other changes to improve the experience on tablets. (I don't see this being a great component for small screens, given the intentional information density of the grid and events, so that won't be the focus.)