This PR takes the batches returned by LightBlue class and stores them in the database. It sets the ArtifactBuild.dep_on for each build to match its dependency on other Docker image according to the data returned by LightBlue.

The state of the builds is initially set to PLANNED. It also adds and sets ArtifactBuild.build_args to json with the data needed to rebuild an image.

The final idea here is that in sign_rpm.py, we start rebuilding the top-level Docker images (those with ArtifactBuild.dep_on set to None). Then we will listen for Brew events informing us that the image has been build. In the handler of that event, we check all the images which depends on just built image (using the ArtifactBuild.dep_on) and trigger their rebuild.