"Reviewer assigned" emails do not respect subscription toggle

Summary

Once a user has been assigned as an MR reviewer, they will continue to receive emails about reviewer assignment even after turning off notifications.

Steps to reproduce

1. Assign a merge request to User A
2. User A receives an email about the assignment
3. User A unassigns self from merge request
4. User A turns off notifications for merge request
5. Assign the merge request to User B

Example Project

This happened to @clavimoniere on !169645 (merged).

What is the current bug behavior?

• User A receives an email notification about the merge request being assigned to User B.

What is the expected correct behavior?

• User A should not receive an email notification

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes