supporter: Refactor amount-checking JavaScript.

* Separate out amount-parsing and reacting into separate events. This sets
the stage for other elements to react to the custom
'conservancy:newamount' event.
* Set up events in the context of each supporter form, with closures. This
lets us avoid weird CSS selector gymnastics in the event, and instead
drill down from the form to find the elements we need.

There are no functional changes in this code, barring bugs.