Incompatible with `db clean`

With this plugin enabled using the core CKAN command to clear the database becomes impossible for a couple of reasons.

1. The tables it tries to delete cannot be deleted until the postgis extension is dropped (`DROP EXTENSION postgis; DROP EXTENSION postgis_topology;`)
2. The clear cannot finish as the extension tries to create missing tables on load, but cannot complete this midway during the clear process.

Start with the core CKAN database-clean command and the plugin's PostgreSQL/PostGIS extension setup described in the issue. Reproduce the failure with the plugin enabled, then trace why table deletion and extension initialization conflict during the clear. Done means the database-clean command completes successfully without requiring manual extension drops.

Written by the indexing model from the issue text.