Add user auditing columns

Currently, there is no record of which account was responsible for changing a dataset or resource, making auditing difficult. There is also no record of how recently an account was used, making it difficult to identify and clean up stale accounts.

Perhaps alongside the modification timestamp there could be a username (or better, UUID).

Start by reviewing the CKAN repository's dataset, resource, and account models to identify where modification timestamps and account usage are represented. The issue does not name files, entry points, or tests, and leaves the identifier choice and scope open; done would require an agreed design plus implementation and coverage for auditing changes and identifying stale accounts.

Written by the indexing model from the issue text.