admin-analytics: add "site.users" "lastActivePeriod" query filter

Created by: erzhtor

Follow-up https://github.com/sourcegraph/sourcegraph/issues/39820.

Test plan

• sg start
• Open API console http://localhost:3080/api/console

fragment SiteUsersFragment on SiteUsers {
  totalCount
  nodes {
    id
    username
    email
    createdAt
    lastActiveAt
    deletedAt
    siteAdmin
    eventsCount
  }
}

{
  site {
    # last active today
    activeTodayUsers: users(lastActivePeriod: TODAY) {
      ... SiteUsersFragment
    }
    # last active this week
    activeThisWeekUsers: users(lastActivePeriod: THIS_WEEK) {
      ... SiteUsersFragment
    }
    # last active this month
    activeThisMonthUsers: users(lastActivePeriod: THIS_MONTH) {
      ... SiteUsersFragment
    }
    # all time
    allUsers: users {
      ... SiteUsersFragment
    }
  }
}