All models: Make sessions_to_include step in base module deterministic

[bill-warner]

Currently sessions greater in length than days_late_allowed (default 3 days) will be included/excluded depending on the update cadence of the model. This is undesirable as the model should be deterministic.

I believe this issue affects both web and mobile models.

Suggested fix.

This fix is yet to be thoroughly tested.

[colmsnowplow]

The suggested fix link leads to the front page of the repo - here's the actual commit.