Activities API to support between two dates

Description

To get active users on a server, it's possible to use the activities API.

We use this in order to count active users on GitLab.com. However, it's not possible to do a 30-day or month-long window between two dates with the API.

Proposal

Allow a to parameter in the query that will return users who are active up until that point. That way specifying: from: 2017-05-01 and to:2017:06-01 will give you active users for the month of May.

Links / references

https://docs.gitlab.com/ce/api/users.html#get-user-activities-admin-only