Adds migration to ensure the new project_mirror_data columns are indeed added to the table

db/migrate/20180503175053_add_missing_columns_to_project_mirror_data.rb

+class AddMissingColumnsToProjectMirrorData < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    add_column :project_mirror_data, :status, :string unless column_exists?(:project_mirror_data, :status)
+    add_column :project_mirror_data, :jid, :string unless column_exists?(:project_mirror_data, :jid)
+    add_column :project_mirror_data, :last_error, :text unless column_exists?(:project_mirror_data, :last_error)
+  end
+
+  def down
+    # db/migrate/20180502122856_create_project_mirror_data.rb will remove the table
+  end
+end