Watch/Improve loadtime performance of POIs

[jmfield2]

Summary:

Now that the new POI system is being used heavily by the layers widget (thanks to the collapsible groups), several new poi queries are requested upon initialization of the web app.

This causes additional data transfer and some extra time to the load time of the app - and could possibly be significant in e.g. certain mobile devices where bandwidth is scarce.

The following screenshots show the before and after network inspector for the POI API queries:

Expected behavior:

Some possibilities for improvements:

1. Investigate enabling CACHING for API endpoint
2. Add a bounding box to the POI API and update the frontend to only load the data for the visible area. NOTE: this will cause additional ajax queries for active layers upon map pan.