Create new MR - [quick actions] Assign reviewers and unassign from reviewers your self doesn't work at the same time

Summary

If you are using quick actions in merge request templates, you can not use /assign_reviewer @user1 @user2 and in the next line /unassign_reviewer me.

Steps to reproduce

  1. Create a merge request template or just add following description:
Some description

/assign me 
/assign_reviewer @user1 @user2
/unassign_reviewer me
  1. Apply this template to the template
  2. Save the merge request

Example Project

What is the current bug behavior?

For example if I am user2, I will still be in the reviewer list.

What is the expected correct behavior?

user2 should not appear in the reviewer list.

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