Adding migration to rename three visas-immigration subsections

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I've opened a PR to add redirects to support these changes - https://github.gds/gds/router-data/pull/138
You call `#save!` here and then `#save` on the next line. You can probably drop this call, unless you want to halt the migration if one doesn't save (which, given the lack of transactions in Mongo, probably isn't what we want).