Reduce duplication in tests
Mark empty contact numbers for destruction

Strong params returns a new hash, meaning that mutating it does not
actually change the main params, meaning we have to change the params
hash diretly. We also need to add the :_destroy param to the
permitted params.
Use common pattern for managing nested attributes

The existing duplicate_fields.js gives us everything we need to handle
the management of nested attributes without the need for custom JS or
param massaging in the controller.
Merge pull request #1410 from alphagov/fix-remove-contact-from-office-bug

Fix remove contact from office bug