Refactor domain package and extract disk serving

Description

This merge request introduces following changes in responsibilities of multiple packages:

• domain becomes responsible for aggregating projects, and exposing a project configuration via Domain.GetProject(...) based on *http.Request
• domain gets information about a project dynamically through a Resolver.
• domain is responsible for creating an instance of a type that implementsserving.Handler, which consists of http.ResponseWriter, *http.Request, *Project and some additional details.
• serving becomes responsible for serving pages based solely on serving.Handler