Update daily notification stats to use 12am utc

[andrewleith]

Summary | Résumé

This PR fixes the daily stats query to use midnight UTC which aligns with the way we handle daily limits.

[smcmurtry]

I think this will return the correct datetime for the case we are trying to fix, but I think there are a couple issues with this change:

• start_date is a timezone aware datetime if limit_days = 1, and it is timezone naive if limit_days > 1. That makes the code harder to understand
• the way start_date is calculated is very different if limit_days = 1 vs if limit_days > 1. This might cause issues in the future when we forget this detail and we are calling the endpoint via notification-admin

Could we just always use utc midnight for this function? I'm thinking something like:

def utc_midnight_n_days_ago(number_of_days):
    """
    Returns utc midnight a number of days ago.
    """
    return get_midnight(datetime.utcnow() - timedelta(days=number_of_days))

in app/utils.py, and then:

start_date = utc_midnight_n_days_ago(limit_days)

in this file.

[andrewleith]

Yeah for sure, I specifically targeted the 1 day case, but you make good points here. Let's chat!

[smcmurtry]

Looks good!