Add option to automatically compute rectangular bounding box

The recommended search backend for ckanext-spatial [only supports rectangular geometries](http://docs.ckan.org/projects/ckanext-spatial/en/latest/spatial-search.html#choosing-a-backend-for-the-spatial-search). In general, however, ckanext-spatial supports more complicated geometries. It would therefore be nice to have an option to automatically compute rectangular bounding boxes (for indexing only) for these.

No file, test, or entry point is identified in the issue. Start by locating the indexing path and the configuration options for spatial search, then determine how supported non-rectangular geometries are represented. Done means an opt-in setting computes rectangular bounding boxes for indexing while preserving the original geometries.

Written by the indexing model from the issue text.