Plan: add DO_FENCE_ENABLE to Copter and Rover mission command lists

ArduPilot 4.5 supports enabling/disabling the fence as part of a mission using MAV_CMD_DO_FENCE_ENABLE and we have a request [here on the Rover-4.5 forums](https://discuss.ardupilot.org/t/change-fence-enable-during-mission/116690) to add it to the Mission Planner's Plan screen's command list.

The request targets Mission Planner's Plan screen and the Copter and Rover mission command lists. Start by tracing how that screen builds its command list and check the existing handling of MAV_CMD_DO_FENCE_ENABLE; done means the command can be selected for missions for both vehicle types.

Written by the indexing model from the issue text.