add Guess Watt game

[dvmarinoff]

Quote from @lawrencemaroun feature suggestion in discussions:

2. A training game called "Guess Watt". The app sets the trainer to a hidden power level, moves it up and down for a bit, and tells > the rider to guess how many watts they are making. Then reveals the wattage and repeats. The goal is to improve one's ability to > estimate their power output, helping with outdoor pacing for those who don't own a power meter.
   Since the first one is not too UI heavy, I can start by writing a JS function to do the workout generation and see how it goes. If it > > seems like a good feature, we can talk about the UI side and how to implement it.