Monthly supporter can enter variable amount.

This required some doing. I'm not completely sure it works, but I
roughly followed the tutorial available at:
https://www.paypal.com/webapps/mpp/get-started/create-recurring-donation-button
with back-reference to this:
https://developer.paypal.com/docs/classic/paypal-payments-standard/integration-guide/Appx_websitestandard_htmlvariables/

My main concern with this setup currently is that 'p3' must be set to
'1', which would seem from the documentation to be saying the payment
will recur only once. There is a subtle hint via the tutorial that
setting 'src' to 1 will override 'p3' with whatever is found in 'srt',
but that's not said anywhere explicitly that I can find. So, I'm going
with this and I'll just test it myself with a monthly subscription to
see if it's indefinite (which is the behavior we herein desire).

Finally, note that "amount" is now a class rather than id, since I'm now
using the associated jQuery .on('input') code for both the annual and
monthly amount boxes.