Allow multiple geometries for each dataset

Current status:
- The `package_extent` table has 2 fields: `package_id` and `the_geom` but `package_id` is a primary key. We cannot add more than one geometry per each package :(
- The `save_package_extent` function check for existing geometries and override them if exists for a package.
- We are using the `spatial` package extra to save the geojson version of the geometries. We `check_spatial_extra` after a package is created or updated.

Start by reading the package_extent table description and the save_package_extent function, then inspect where check_spatial_extra is called after package creation or updates. Define how multiple geometries should be stored and preserved per package, and verify that saving a package no longer overwrites an existing geometry.

Written by the indexing model from the issue text.