Refactor MigratorLEI into Migrations

[tiredpixel]

Refactor RegisterSourcesBods::Services::MigratorLEI into the new RegisterSourcesBods::Migrations method.

This is quite a small task, and the LEI migrations have already been run, but I think there's merit in refactoring and importing it, not only to clean up existing code, but also as a useful reference for how to migrate identifiers.

It should also include refactoring RegisterSourcesOC::Repositories::AddIdRepository search_scroll to use since-added RegisterCommon::Elasticsearch::Query search_scroll instead, further reducing duplication (one function at a time…).

• RegisterSourcesBods::Services::MigratorLEI
• RegisterSourcesOC::Repositories::AddIdRepository

This was noted as part of #222 .