Reward Users for Keeping their plants healthy

[UPstartDeveloper]

TODO: see how we can generalize an algorithm to reward people whose plants are marked as healthy.

Should take into account:

1. Differences in how much CO2 a plant sequesters
2. Differences in how frequently a person checks their plant health
3. Once this is done, we ought to also add it to wherever we publish the rest of the scoring rules in #131