Allow different periods for different repos

[damithc]

Now that we have a 'trim timeline' feature, and also working towards a 'code portfolio' use case, perhaps it is also time to support different time periods for different repos.

This can cut down processing time, especially for code portfolios use case.

However, this is not needed urgently, and I expect will need drastic changes to the implementation.

Still, I'm posting it here for future reference, as something we can tackle when we are ready.