Refactor board_scope and board_configuration_options to stop mutating props

Currently, ee/app/assets/javascripts/boards/components/board_scope.vue and app/assets/javascripts/boards/components/board_configuration_options.vue components mutate the board prop, using an object reference. This breaks a unidirectional flow of data, breaks component isolation and goes against Vue documentation: if we need to change a prop, we emit an event to the component 'owning' this property (in this case, to app/assets/javascripts/boards/components/board_form.vue