Data Filter Menu Design: Implement legend headers

[mcdonoughsh]

Allow for categorizing of data layers in the Explore page legend/data filter menu:

• Allow for items in the legend to be nested under a header.
• Allow for nested data layers to be minimized and expand when header is clicked
• when considering interactivity, consider design (add + or - so that user is aware they can expand/minimize)

Data from Powerful Pathways

Land use -- Understanding how the land in Mattapan has been designated -- this shows what type of housing exists where, vacant land that could become community-owned, where 
      Parcel use
      Section 8/affordable Housing
Public Space/Amenities
      Parks and green space
      Local landmarks
Transportation  
      Public transportation
      Blue bikes
      Parking map
      TAZ
      Census data on commuters
311 calls

[laudickson]

I'll talk about some stuff in #89 and I think this will fit in nicely