Support curated map stories

[goldpbear]

We'd like to add the following additional functionality to the story map feature:

• Provide users with a story overview -- #502
• Performance optimizations -- #495
• Places in multiple stories simultaneously -- #503
• Configurable page and place detail heights -- #548
• Dynamic "explore stories" button -- #547

[goldpbear]

Note on panning controls: we can now set custom center points for individual story elements. If a custom center point is defined, the map will center to that point; otherwise, the regular center point for that place/landmark will be used.

Config setup for a custom center point looks like this:

order: 
  - url: marra
    panTo:
       lat: 47.53722
       lng: -122.32573
  - url: roxhill-bog

[goldpbear]

@futuresoup -- I put together a few mockups of what our story overview feature might look like. Clicking on an icon in the overview bar would open that place/landmark in the sidebar, and scroll the overview bar to center around that icon:

[goldpbear]

A few more mockup ideas:

[goldpbear]

A couple more ideas: