Add calendar heatmap showing exercise history

[brylie]

The calendar heatmap would be a good way to show the history of workouts and improve consistency. However, it shouldn't encourage people to over-exert themselves. So, it should be a "consistency calendar" or something similar. This means it should be based on a timespan of several days to determine the color, rather than a simple count or sum of activity in a given day. This is basically like a Goldilocks zone for proper balance.

Possible colors:

• red: over exerting (many exercises within a few days prior)
• green: regular engagement (not too much, but not too little)
• blue: under-exerting (no exercises within a few days prior)