* Rename PaymentRequest.value -> requested_value

* Add PaymentRequest.paid_value

* Expose "actual" value of a PaymentRequest

"Actual" value is paid_value when it exists or requested_value
otherwise.  This adds a property so it can get used with an instance and
updates the payments block QuerySet to let the database pick the right
values.

* Fix displaying form errors in messages

This sends one message per error and no longer uses Django's error
markup since our styles rendered the messages poorly.

* Update comment when changing PaymentRequest status