Data about activity of user base in an instance

This issue is the working issue for https://gitlab.com/gitlab-org/gitlab-ce/issues/3962

Description

We need to provide a new column in the database, last_activity_at, linked to a user, showing the last time this user did something with the system, either log in to the UI or do some git action via ssh or http.

That way, administrators of an instance can query the DB to know who's active, and who's not.

References

• This issue comes from concerns raised in https://gitlab.com/gitlab-org/gitlab-ce/issues/3962
• If done, it will solve https://gitlab.com/gitlab-org/gitlab-ee/issues/545

cc @JobV @dblessing