Make model issuer aware

[lukpueh]

The Project class added in #2, and PIA in general, are unaware of whether a project uses GitHub or Jenkins. The expected issuer is defined dynamically via the issuer field, and expected token claims are defined via an opaque <claim name>:<claim value> dictionary field.

It might make sense to create subclasses, e.g. GitHubProject and JenkinsProject to:

• implement stricter project config schema validation on model level , e.g. GitHubProject, i.e. a project with a GitHub-specific issuer, must have repository in its required claims
• implement static issuer-specific checks (see warehouse's GH publisher)