Data Filter Menu Design: Implement legend headers

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
```