Deactivated users still receive emails from filter subscriptions

Summary

Filter Subscriptions will still send emails to users that have been deactivated.

Steps to Reproduce

1. Create a test filter
2. Create a new user that you can access the instance as.
3. ensure the user has the permissions required to access the test filter
4. log in as the user and subscribe to the filter. Set the time to every 15 minutes to make it trigger rapidly, and set it to send even if the filter results are empty. This will help work around permissions issues in trying to reproduce the issue.
5. deactivate the user by using the "revoke site access" option from the "..." menu in the user search
6. Wait for the next filter subscription trigger. You may need to wait for a couple of cycles, so do something else and then check the inbox later. The emails should be there.

Expected Results

No email would be sent.

Actual Results

Emails are sent.

Workaround:

Currently, the only way to solve this issue is to reactivate the user, log in as the user in question, then browse to the filter and remove the subscription. You can then deactivate the user again and the emails will no longer be sent.