Set a weekly survey limit

Add a weekly count of surveys shown, and prevent showing a new notification if we've already reached that limit.

Simplifies and renames the setSurveyCount functions, and adds a new alarm to reset the weekly count every seven days at midnight.

This fixes issue #88 
For the daily alarm, this was to make the alarm set for the *next* midnight instead of last midnight. But I realize there are two issues here:

1. With a period of a day, this shouldn't matter anyway.
2. We could more clearly achieve this by doing `midnight.setHours(24, 0, 0 ,0)`.

For (1), I tested this, and we can just get rid of the added milliseconds entirely. The worst edge case is that Chrome sees a non-conforming "when" (less than 1 minute from now) and schedules it sometime in the near future, but the period should still work.

I pushed a commit that just gets rid of the added milliseconds entirely.