Create list for an issue board, cannot assign to person that is member of private group invited to project

Summary

First reported in #36069 (comment 1430470822)

This is "Create list for an issue board" - it cannot find people outside of your group.

Steps to reproduce

1. Create a public project
2. Go to Members page. Invite a group, select a private group (this group should not be part of the group hierarchy)
3. Add some users as a member of the private group (say bob)
4. Login, or impersonate as a member of the public project
5. Create a new issue in the project. Try to autocomplete @bob in the issue description.

Example Project

What is the current bug behavior?

User from private group invited to project does not appear in autocomplete

What is the expected correct behavior?

User from private group invited to project appears in autocomplete

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