Create database when starting site

If the database connection fails when starting a site, try to create the database. This fixes #838. 

@arjan @mworrell I’m currently not using the `z_db` database wrapper correctly but I call `pgsql` functions directly instead. The problem here is that the connection fails when the database does not exist, so I have to create my own connection. I didn’t get it to work using the `z_db`/`z_db_pgsql` modules. Can you help out?

TBD:
- [x] Create schema, too.
- [x] @mworrell Is `CREATE LANGUAGE` no longer required? Seems to work fine without it.
- [x] Docs.