Blocked user still receiving notification e-mails

Summary

Blocked users still receive notification e-mails and they do not have an option to get rid of these notifications since they can't login.

Steps to reproduce

1. Create a new user
2. Create a MR/issue and assign it to this user - by default he should receive notifications for this issue now
3. Make a comment on the issue to verify the user receives notifications
4. Block the user in the admin panel
5. Make another comment on the issue created in 2.
6. Blocked user will receive a notification e-mail

Example Project

n/a

What is the current bug behavior?

User is receiving notifications although they have been blocked

What is the expected correct behavior?

Intellectual property and other private information shouldn't be sent out to third parties.

Relevant logs and/or screenshots

n/a

Output of checks

It happens on EE - can't verify on GitLab.com since I don't have access to the admin panel ;)

Results of GitLab environment info

GitLab Enterprise Edition 10.0.2-ee f5b3271d

Results of GitLab application Check

n/a

Possible fixes

When sending notifications, verify the account is still active.