Implement parameterized payout / incentives description and calculation

[cjqian]

From the tech doc:

Payment information for the user who did the experiment.
Lucas: prototype something so we have a payment page.
Follow. General guidelines. ~10 EUR/hour.
Have a %increase if you win (selected leader & for getting the right value).
TODO(stefano.palminteri@gmail.com Maël Lebreton): formula for how much we pay participants.

Moving forward, we should think about how to calculate payouts as a general experiment feature (can we untangle this logic from the Lost at Sea-specific logic?)