Add support for model object anonymization
Simply add a BaseModel.anonymize() method. Default implementation returns None, meaning the object is not anonymizable.
For Revision, Release and Person, the method do return an anonymized version of the object.
Replaces (partially) the couple swh-journal!172 (closed)/swh-storage!400 (closed).
See swh-journal!173 (closed) for the part in swh.journal.
Migrated from D3171 (view on Phabricator)