Scale throttle outputs by battery level. Fixes #4751

msg/battery_status.msg

src/modules/mc_att_control/mc_att_control_main.cpp

src/modules/mc_att_control/mc_att_control_params.c

src/modules/sdlog2/sdlog2.c

src/modules/sdlog2/sdlog2_messages.h

src/modules/systemlib/battery.cpp

src/modules/systemlib/battery.h