lister.gnu: Standardize arguments to pass to the loader tar

This:

• Moves version parsing logic from loader-gnu to lister
• Parses the timestamps provided by gnu tree to isoformat dates (to provide to loader)
• (Aligns the timestamps stored in the lister model to be also a date in the right format to clarify reading from it)

This allows to have a generic "tar" loader. That way, the loader expects some required arguments in the right type already. No need for it to try and do specific computations depending on the origin of the archives.

Related !385 (closed)

Note: This actually implies a schema migration (timestamp model migration) As the loader gnu never ran though, we might as well drop the existing data and trigger a new run when time comes. So it's not a blocker.

Test Plan

tox

Migrated from D2147 (view on Phabricator)