Storymap walkthrough feature

[futuresoup]

The goal is to be able to navigate though various map features like a slideshow i.e.
https://storymap.knightlab.com/

For now we can just have an easy config setting:

Story:
  id: cleanup-sites
  order: reference-url1, reference-url2, etc.

For now we can start it as a javascript array, but in the future we likely want it to be in the actual django data model itself

[goldpbear]

Some thoughts on improvements to the story feature:

• Have two sets of story controls on a place detail view: one at the top of the view, and one at the bottom
• Use larger arrow buttons
• Use left and right chevrons instead of arrows
• Add the ability for stories to turn map layers on and off, the idea being that stories will be able to control the layers that are relevant to them, giving the user a more focused browsing experience
• Make stories loop
• Incorporate design changes in use on the gtopenspace flavor into the base story feature
• Add the ability to control zoom levels for all story elements
• Return map layer configuration to default on story close
• Maybe add a story sidebar feature that summarizes elements of a story
• Add map panning via story controls
• Zooming will hide the spotlight -- let's make sure it stays around when the map is zoomed