entities.rb

To allow jobs/builds to have multiple artifacts, and to start seperating
concerns from Ci::Build a new model is created: Ci::Artifact.

Changes include the updating of the ArtifactUploader to adapt to a
slightly different interface. The uploader expects to be initialized
with a `Ci::Build`.

Futher a migration with the minimal fields, the needed foreign keys and
an index.

Last, the way this works is by prepending a module to Ci::Build so we
can basically override behaviour but if needed use `super` to get the
original behaviour.