Check that postgis extension is available on init

When doing init, it would be good to do a check in the database to make sure that postgis extension is enabled - probably something like `select extname from pg_extension;`

if postgis is not available in the database, we could try to add it with `create extension postgis;` - and if that fails, best to report an error (that may need higher level of permissions, or even install some packages)