pay: UX improvements on payment history page

[dehnert]

With PR #47, we added support for showing people's payment history (#43 ). There's some UX improvements we could make though:

• The list appears to put the earliest payment at the top. I think people expect to see their latest payment at the top.
• We display quite limited information about payments -- basically we're limited by what the Transaction object has. Incorporating more information would be useful. For example:
  ** Payment/transaction amount (which isn't especially well defined -- amount of any credit card line item? sum of all positive line items?)
  ** Individual line items? The Amazon order page, for example, will show this (though not in much detail)