Alter generic leaderboards to have a "sponsor" field

[obscurerichard]

Many of the leaderboards defined in freezing-web/leaderboards have a sponsor defined. It would be good to have both a sponsor display name and URL in the template for this so that:

1. We maybe don't have to keep changing these year after year, and
2. To give people a link to the sponsor if they want to ask questions

If there is no sponsor, the leaderboard should have a generic message, for example it should say "Currently this pointless prize is unsponsored, please speak up on the forum if you want to sponsor it."

[obscurerichard]

This is partially implemented now, but we don't have a link to the sponsor on the forum, and no default text if it has no sponsor.