Generalize rating initialization based on another rating

[Askaholic]

For 2v2 we wanted players ratings to be initialized based on their global rating, but with a sigma increase. This was hardcoded into the player rating dictionary here:

server/server/rating.py

Lines 39 to 47 in d2265dc

	if key == "tmm_2v2" and key not in self:
	mean, dev = self[RatingType.GLOBAL]
	if dev > 250:
	tmm_2v2_rating = (mean, dev)
	else:
	tmm_2v2_rating = (mean, min(dev + 150, 250))
	self[key] = tmm_2v2_rating
	return tmm_2v2_rating

For future 3v3+ queues we will want to do a similar thing, so it would be best to fully generalize this process. We should add a field in the database to the leaderboard table that will reference another leaderboard id to initialize from. I think we can leave the sigma increase as it is, hardcoded to +150 sigma with a 250 sigma cap. I don't see a reason why those values would need to be configured in the database; they would only complicate the tables.