Minor adjustment to delay windows

[csharrison]

Currently, the event-level API has a series of deadlines which slot reports into various reporting windows. See spec. Right now, e.g. triggers that are registered within (2d - 1hr) are reported at source_time + 2d. This was designed to impose a minimum of 1 hour delay.

This design is fairly unintuitive. It complicates the question of "what reporting window does this report slot into", while not really simplifying the question of "when will a report in this window actually be sent" (due to other delays like network availability, browser running, etc).

I propose we adjust the obtain early deadlines algorithm to return exactly 2d, 7d early deadlines for navigation sources.