Add helper for renaming columns using a background migration

We have a helper for changing column types using a background migration: https://docs.gitlab.com/ce/development/what_requires_downtime.html#changing-column-types-for-large-tables

We should:

1. Add a similar helper for renames (the existing helper would almost work unchanged, as the background migration is generic enough to support it).
2. Document that helper on the same page.
3. Update https://gitlab.com/gitlab-org/gitlab-ce/blob/master/rubocop/cop/migration/update_large_table.rb to also detect rename_column_concurrently and change_column_type_concurrently, to remove any element of human error.

/cc @yorickpeterse @abrandl