Finish any remaining jobs for issues.closed_at

This ensures that all rows in the issues table use timestamp with time zone for the closed_at column, even if Sidekiq jobs were lost. For GitLab.com all data has already been migrated so this migration will be a no-op.

Migration Output

== 20180201145907 MigrateRemainingIssuesClosedAt: migrating ===================
-- columns("issues")
   -> 0.0012s
== 20180201145907 MigrateRemainingIssuesClosedAt: migrated (0.0067s) ==========

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

General Checklist

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together