Schedule workers to delete non-latest diffs in post-migration

What does this MR do?

This MR schedules workers for deleting non-latest MR diff files. It's part of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19670.

PG query plans on staging were commented directly on the code

Migration output

== 20180619121030 EnqueueDeleteDiffFilesWorkers: migrating ====================
-- indexes(:merge_request_diffs)
   -> 0.0018s
-- transaction_open?()
   -> 0.0000s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- index_exists?(:merge_request_diffs, :id, {:where=>"(state NOT IN ('without_files', 'empty'))", :name=>"tmp_partial_diff_id_with_files_index", :algorithm=>:concurrently})
   -> 0.0009s
-- add_index(:merge_request_diffs, :id, {:where=>"(state NOT IN ('without_files', 'empty'))", :name=>"tmp_partial_diff_id_with_files_index", :algorithm=>:concurrently})
   -> 0.0196s
-- transaction_open?()
   -> 0.0000s
-- select_one("SELECT current_setting('server_version_num') AS v")
   -> 0.0005s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- indexes(:merge_request_diffs)
   -> 0.0013s
-- remove_index(:merge_request_diffs, {:algorithm=>:concurrently, :name=>"tmp_partial_diff_id_with_files_index"})
   -> 0.0031s
== 20180619121030 EnqueueDeleteDiffFilesWorkers: migrated (0.0801s) ===========

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/37639