Reorganize leaderboard medals to account for modern higher user counts.

The site has grown quite a bit since leaderboard medals were introduced: Over 1000 people have uploaded at least two Pony models in the last 30 days. I think the old brackets aren’t really applicable anymore. I propose One: moving to the following breakdowns
Top 1: Platinum
Top 10: Gold
Top 100: Silver
Top 250: Bronze
And two: Making access to a badge for high ranking monthly leaderboard placement permanent (or, at least, if claimed). Could be a separate badge from the active leader, but badge disappearing once you fall off doesn’t really incentive using them or seeking them.