this includes a migration and much more to be listed individually

BREAKING CHANGE:

Closes: #65

Instead of log levels, tank just logs info messages to stdout, error messages to stderr and access logs as configured.

Now it logs info messages to `info.log`, error messages to `error.log`
and access logs to `access.log` inside `/srv/logs/`.

Closes: #55

* based on 2dba85db
* build with `make api-docs`
* Swagger UI available at `/api/docs/index.html`