Subscription payment enhancements

[dehnert]

It would be useful to:

• Track a creation time for payments as opposed to payment time -- in particular, credit card payments we have a payment time from the server, but tracking the import time would sometimes be useful
• Admin should allow filtering by payment type
• For gate, it would be nice to track who was handling gate at the time, rather than just the signin user account

[dehnert]

It would be useful to:
[...]
* Admin should allow filtering by payment type

Fixed in commit 16069e6.