[bug 971782] JS fix for data bucketing (weekly, monthly).

The bucketing implemented in Rickshaw does not like missing data
for any dates. It wants explicit 0s. This zero-fills all the
dates.

To verify, go to http://localhost:8000/en-US/questions/dashboard/metrics/en-US?product=firefox-os, change the dropdown to weekly and make sure that it's not a flat line on 0 like in prod. You might need a recentish db dump, but it doesn't have to be the latest.

r?
for sure