Enable story thumbnails that reflect styling in the story

[jonpmarino]

Right now, when a user publishes a MapStory, the story uses an already-existing thumbnail from a layer in the story as its story thumbnail. This was always a sub-optimal stop-gap until we could create a workflow for generating story thumbnails that provide some indication of what is in the story itself and differentiating them from layers.

This is will demand some R/D to generate options.

One workflow that would mimic what we used to have on the original mapstory.org is to have the user publishing the story use a cropping tool of some kind to manually generate the frames for the thumbnail.

Or, perhaps when "publish" is clicked, a process is triggered to snap a few images of the story's Chapter 1 and produce the thumbnail.