Can't add thread on merge request during review

Summary

It's being added as a simple comment instead, so the merge request can be incorrectly merge without solving that review comment.

Steps to reproduce

• Go to a MR
• Start a review by leaving a message on a line of code
• Go back to the Overview tab
• Write a message
• Click the new "Add to review" button that shows up
• Submit the review

Example Project

What is the current bug behavior?

A simple message is sent.

What is the expected correct behavior?

A thread should be sent instead, because, knowing that it's a review, it:

• should block the merge until it gets resolved
• should allow replied

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

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