Make user deactivation emails opt-in

Sending an email to users when their account gets deactivated sort of defeats the purpose for those instances where deactivating users is used in an effort to control Gitlab license consumption.

In our case, there are people who may interact very rarely with Gitlab. Maybe a HW engineer needing to update a schematic once every quarter or so. Or hardware debug engineers that create issues very infrequently.

Since they're legit users of Gitlab they shouldn't be blocked so that there is no administrative overhead of blocking and unblocking these infrequent users.

We may have 80 of these users who can need access at a moments notice. Since Gitlab uses a high watermark license model, it makes sense for these users to be deactivated until they actually need to use a license. No sense paying for 80 licenses when maybe only 10 are needed for a quarter.

Firing an email notification to these users will likely cause them to log back in and spike the license usage for a given instance. And this will happen every quarter when they get deactivated again due to inactivity... Meaning we'll see no real savings for auto deactivating users. The waiting period for deactivation was dropped to 90 days from 180 due to customer demand for these exact scenarios. See #201769 (closed). #297416 (closed) !64964 (merged) will undermine some of that work.