Introduce finish_rules

There are two instances of potential circular reasoning in `grading_rules`:

* Which grading rule gets used may depend on the completion date, by way of `if_completed_before`. On the other hand, the completion date may depend on
* Whether a flow gets finished may depend on the due date. But the due date is part of a grading rule, which can't really be evaluated before a flow is finished, given the existence of conditionals like `if_completed_before`.

In the current code, this is resolved in an ad-hoc manner, *somewhat* sensibly--the conditional uses the current time as the completion time when choosing grading rules for in-progress sessions, or, as of af46e5f222896305a10a366fe783001dcc0073ab, also the time of the last activity if `use_last_activity_as_completion_time` is set.

A reasonable way to resolve this would be to introduce another set of rules (maybe `finish_rules`) that get evaluated before a flow is finished (to set things like the due date and what should be used as the completion time).

cc @eecarrier