Add relationship between blobs and manifest_configurations

Problem

After !233 (merged), we're now saving layer and configuration blobs in blobs (previously it was named layers). In order to avoid data duplication, as we do with manifest_layers, we should not save the configs media_type, digest and size in manifest_configurations, as those attributes are already part of blobs.

Solution

Just like manifest_layers have a blob_id FK, manifest_configurations should have one as well. For convenience, models.ManifestConfiguration still has the MediaType, Digest and Size attributes, and we source those from blobs with a JOIN whenever we read configurations from the database. As a positive side effect, this change now enforces the existence of a config blob (blobs) before a config object (manifest_configurations) can be created.