Consume remaining LinkLFsObjectsProjects jobs

In 12.9, a background migration to link LFS objects from source to
forks has been added.

As part of cleaning up process described in
https://docs.gitlab.com/ee/development/background_migrations.html#cleaning-up,
to ensure that all jobs ran and records were created, this post
deploy migration will run the `LinkLfsObjectsProjects` background
migration job synchronously via
`Gitlab::BackgroundMigration.steal`.

changelogs/unreleased/209035-consume-link-lfs-objects-projects.yml

+---
+title: Consume remaining LinkLFsObjectsProjects jobs
+merge_request: 27558
+author:
+type: other

db/post_migrate/20200319071702_consume_remaining_link_lfs_objects_projects_jobs.rb

+# frozen_string_literal: true
+
+class ConsumeRemainingLinkLfsObjectsProjectsJobs < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    Gitlab::BackgroundMigration.steal('LinkLfsObjectsProjects')
+  end
+
+  def down
+    # no-op as there is no need to do anything if this gets rolled back
+  end
+end

db/structure.sql

@@ -12901,6 +12901,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200318164448
 20200318165448
 20200318175008
+20200319071702
 20200319123041
 20200319203901
 20200320112455