Add a new trac.initdb command to create a default empty trac database

It drops the postgres cluster, recreates it and generates the trac default database. It also performs that routine when installing trac.

This depends on https://github.com/twisted-infra/braid/pull/63 to be merged first.

This is the second part of a fix for https://github.com/twisted-infra/braid/issues/54