Always store checkboxes as arrays in state#270

alexluckett wants to merge 3 commits intomainDEFRA/forms-engine-plugin:mainfrom

bugfix/DF-686-dont-store-checkboxes-as-singleDEFRA/forms-engine-plugin:bugfix/DF-686-dont-store-checkboxes-as-singleCopy head branch name to clipboard