Implement simple_bbox option for datasets endpoint

[brlodi]

A simple_bbox flag was added to the shape endpoint in 41abdfa to allow quicker responses when query geometry intersection with the entire dataset is important but the individual shapes are not. Setting the flag to true emulates the current behavior of the event endpoint, which only checks the bounding box of the complete dataset. It would be nice to have similar opt-in functionality on the datasets endpoint, so users willing to wait for the more in-depth query have a method of filtering false positives (e.g. a query area in the Midwest intersects with the bbox of a dataset with events in LA and NYC but does not actually contain any events itself).

It would make this a breaking change, but the opt-in should be the same in both cases, i.e. /datasets should default to in-depth search with an optional simple_bbox flag like /shapes does, or probably better yet both should default to a simple search with some sort of strict_contains flag.