fix: calculate and show accurate count of max users per license

Created by: dadlerj

This PR updates the CHANGELOG.md file to describe any user-facing changes.

This fix makes it so that:

• We now show the correct number of max user accounts on an instance via the GraphQL field Site.productSubscription.actualUserCount
• We expose a new GraphQL field Site.productSubscription.actualUserCountTime indicating the time when the max count of users was hit. This now appears on the site-admin license page, when you hover over that number.
• We run a background routine every 6 hours that calculates the number of active user accounts. I chose to have this happen in the background, rather than continuously when new users are added, because (1) it simplified things a bit (e.g. made it so I didn't have to call the enterprise-only licensing package from db), and (2) it also gives admins a bit of slack — they get a bit of time to delete rogue accounts if they go above their license.