Allow dynamic symbology classification to be filtered by geographic entity

[torrin47]

In the MVP of the symbology widget, classification statistics are always drawn from the entire underlying dataset. The user also needs to be able to limit the statistics used for classification to a specific subset of the data:

• Visible bounding box - (would this be dynamic and reclassified on every pan and zoom?)
• Fixed bounding box - set once and pinned to that bounding box until unpinned
• Community (would this only be for community layers? What about layers calculated for HUC12? Would we also assign these to a community?
• State?
• Region?
• Uploaded shapefile?

[torrin47]

This issue was moved to USEPA/EnviroAtlas_JSApp#12