Add activities from an indoor trainer

[noelbruin]

Would be wonderful to have activities that were recorded on an indoor trainer, but only if it has an accurate power source.

I think you could use the "device_watts"=true as the way to confirm if the trainer ride comes with power reported from an actual power meter and also the "trainer" field will be needed.

This could then enable all sorts of wattage related jerseys. highest average watts, highest watts overal, highest watt/kg etc.