Add user activities API

Adds API methods to query for user activity (saved in redis). The data should be paginated and restricted to admins only. Also added an option to filter by date.

Example:

GET /user/activities?page=2&per_page=50&from=2016-03-11

Should output a list of usernames and last activity timestamps.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/1311