Per-realm configurations

[thomax]

Add support for .yml configs on a per-realm basis.

These should support both s3 bucket configurations and a manifest of image sizes for a realm + application.

[thomax]

Copied from #14 (comment)
@bjoerge @simen @thomax Do we still want to do this? If we're open sourcing this, do we need to have a different type of persistence? (we could look at pstore, for instance, for a more lightweight approach than psql).