[SCH-1975] Add rake task to get docs from one index and check against another

[stringiest]

Adding a rake task to retrieve content_ids for all docs in one index, and then to check another index to ensure they exist there. Intended to be used to check that all docs from government index were copied over to govuk index.

WIP:

• added initial attempt at search query to get all docs
• Need to set a batch size then re-request until all docs are retrieved
• Haven't checked yet whether digging gives correct output.