Transactions by channel migrations

Adds a method `migrate_data_set` which will take an old data set config, override specified parts of it and create a new data set with this specification (or update an existing one that matches this identifiers of the exising one). Then it will load the old data sets data using the backdrop client, convert the data to a new format defined by `data_mapping` and post it to the backdrop data-set for the newly created data set config.

Adds migration to use this for two data sets

Carers allowance monthly has been left uncombined with carers allowance weekly as it is no longer used.

Do not merge until we have information from cliff about changing values as well as keys in the data for carers allowance weekly.
Can this comment be removed now?