feature: switch to sort pledges by age or size

If a sorting filter can be setup discretely and intuitively, it could provide these benefits:

• Make it easier and more engaging to monitor a campaign (sort by latest)
• Make it easier to incentivize pledges other than "largest" (e.g. "earliest" or "most recent")
• Generally allow people to see the campaign the way they want to see it.

As @monsterbitar points out below in the comments, it may also somewhat disincentivize large donations since currently they have a clear and fixed place at the top of the list.