Post-deploy to clean `pending_delete` projects

What does this MR do?

There are many projects in pending_delete state, this post-deploy migration cleans them up.

The script is based on https://gitlab.com/gitlab-org/gitlab-ce/snippets/1648654 and https://gitlab.com/gitlab-org/gitlab-ce/snippets/1611429. The use of these scripts were described in https://gitlab.com/gitlab-com/infrastructure/issues/888.

Are there points in the code the reviewer needs to double check?

Should this be done in a background worker?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#20984