Give Humbug integrations ability to present users with reports when asking for consent

[zomglings]

Consent is checked on every report publication anyway, so it makes sense for consent.ConsentMechanism functions to take an optional Report argument. This can be used to present users with the contents of the report for approval, so they can see exactly what they are consenting to.

Future flows along these lines: users are presented with a raw report and they can modify it prior to giving consent.

Something to think about, the only change required here would be to change the definition of ConsentMechanism from Callable[[], bool] to Callable[[Optional[Report]], bool].

As it stands now, this would introduce a circular dependency between humbug.consent and humbug.report, so we would have to move Report into another file (e.g. data.py).