LP#1749992 Disable payment button during payment

LP#1749992 Disable payment button during payment

Since the display is not refreshed until the payment finishes, it
is possible to fire off a second payment by double-clicking.

This commit is a simple way to disable the button while a payment is
in process, then enable it again after the display refreshes.

Since the 'sendPayment()' function is wrapped, we may need to move
this logic up, but this being a single promise is simpler, and it
solved the problem in testing.

Signed-off-by: Dan Wells <dbw2@calvin.edu>