feat: allow for adding extra databases

Allow for declaring extra databases that should be created when the
project is first started. This is done by adding an `extra_databases`
key to `database`, containing a list of database names.

```yaml
database:
  type: mariadb
  version: 10
  extra_databases:
    - tinhat
    - freshcover
```

templates/docker-compose.yaml.twig

@@ -82,6 +82,7 @@ services:
           memory: "${DOCKER_MYSQL_MEMORY:-0}"
     volumes:
       - db-data:/var/lib/mysql
+      {{ database.extra_databases is not empty ? '- ./tools/docker/images/database/root/docker-entrypoint-initdb.d:/docker-entrypoint-initdb.d' : '' }}
     env_file:
       - .env
     labels: