Add a creation timestamp offset for default content

Problem/Motivation

In 🐛 Umami content is all created in the same second Active we realised that all Umami content is created in the same second, which is not very realistic.

Could we add something like a timestamp offset to default content imports, so that content can be spread out a few hours/days back as it gets imported? Would have to be a special key which then gets applied as an offset to the creation time.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet