Create fork networks for deleted source projects

This adjusts the PopulateForkNetworksRange background migration to create fork networks for forks for which the source was deleted.

This allows the forks of those fork to be included in the same network, allowing them to still send merge requests to each other.

It also solves the problem described in https://gitlab.com/gitlab-org/gitlab-ce/issues/40072#note_48335746 in which the background migration to create the fork network memberships for fork of forks would be rescheduled indefinitely.

Database Checklist

When adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted
• Added the output of the migration(s) to the MR body

== 20171124150326 RescheduleForkNetworkCreation: migrating ====================
-- Populating the `fork_networks` based on existing `forked_project_links`
== 20171124150326 RescheduleForkNetworkCreation: migrated (0.0186s) ===========

• Added tests for the migration in spec/migrations if necessary (e.g. when migrating data)

General Checklist

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/40072