API for daily active users metrics

[smirolo]

We will count as a daily active user one where start_of_day(yesterday) < User.last_login < end_of_day(yesterday). This will be used in dashboard widget:

• This API endpoint could also be used to show a graph of daily active users through time. So it could/should be modeled on the model of saas.api.metrics.RevenueMetricAPIView for both request parameters and result JSON schema.

• The definition of start_of_day/end_of_day should take into account timezones to make sure metrics do not change based on where the browser making a request is located.