CI/CD for external repo doesn't automatically create webhook

Summary

When a user successfully configures GitLab CI/CD for external repositories the webhook on the external side is not created, so CI/CD does not work when a change is introduced.

Steps to reproduce

  1. Create a new project on GitLab.com and configure CI/CD for external repositories.
  2. Make a change in the external repository and create a pull request.
  3. Observe that a pipeline is not initiated in the pull request.
  4. Go to the webhook settings for the external repository and observe that one hasn't been created.

Example Project

Replicated here mirrored from this GitHub repo.

What is the current bug behavior?

A webhook is not created on the service where the repository originated causing CI/CD to not work.

What is the expected correct behavior?

It is expected that a webhook is created so that GitLab CI/CD will work successfully on the external repository.

Relevant logs and/or screenshots

The created webhook on GitHub should look like this:

ci-gh-webhook

Currently, when a pull request is opened on the external repository this message is seen due to there not being a webhook:

cicd-on-github

Output of checks

This bug happens on GitLab.com: 12.0.0-pre ec198c44beb

ZD: https://gitlab.zendesk.com/agent/tickets/124395 (Internal)

/cc @jlenny