The utilities for serialization, deserialization, and generation of

commit validation metadata have been moved into the commit_utils

module. Previously, the (de-)serialization logic was implemented in a

multitude of places and the generation was intrinsically tied to the

ValidateDiffCommitResource.
However, we will soon require validation metadata in the

DraftDiffResource for finalization of commit series, and so to not

reimplement this all in another place, it has been refactored into the

diffviewer.commit_utils module.

Ran unit tests.