Develop a new major version

ckanext-spatial is an old extension and hasn't had a big refactoring in a long while. Postgis and other python requirements make it difficult to install and deploy. In addition, the [spatial search capabilities](https://solr.apache.org/guide/8_11/spatial-search.html) in Solr have evolved significantly since the current implementation and both indexing and querying can be greatly simplified in ckanext-spatial.

The main areas of work will be:

* Dropping PostGIS requirement (#195 ). Support only Solr-based search (the PostGIS one is not suitable for production anyway)
* Refactor search logic. Use modern Solr fields to support the existing backend modes (`solr` for bboxes and ` solr-spatial-field` for polygons, points, etc), they can be probably merged into a single implementation. Write actual Solr based tests. See #291
This work could be tagged as `v2.0.0`, and ideally will only target CKAN 2.9+ in Python 3, although we can re-assess this when we know the scope of the changes

cc @avdata99