Merge branch 'dz-improve-ci-id-migration' into 'master'

Add extra checks for ci_id migration

See merge request gitlab-org/gitlab-ce!28404

db/post_migrate/20190424134256_drop_projects_ci_id.rb

@@ -22,7 +22,12 @@ class DropProjectsCiId < ActiveRecord::Migration[5.1]
   end
 
   def down
-    add_column :projects, :ci_id, :integer
-    add_concurrent_index :projects, :ci_id
+    unless column_exists?(:projects, :ci_id)
+      add_column :projects, :ci_id, :integer
+    end
+
+    unless index_exists?(:projects, :ci_id)
+      add_concurrent_index :projects, :ci_id
+    end
   end
 end