Track dependencies for automatically triggering pipelines

We need to track dependencies for automatically triggering pipelines, follow up for https://gitlab.com/gitlab-org/gitlab-ee/issues/9045.

This should be done in a separate database table.

Kamil's thoughts - https://gitlab.com/gitlab-org/gitlab-ee/issues/9045#note_159388649

TL;DR:

ci_pipelines_dependent_projects:
  - source_project_id # upstream:project
  - source_ref # full qualified ref-name, like `refs/heads/master` or `refs/heads/tags`,
  - target_project_id # downstream:project
  - target_ref # full qualified ref-name, like `refs/heads/master`, today it will be only one
  - target_job_name # the name of the job holding this reference? maybe we don't need that