Float Donate button to the center beside questions

The donate button is now centered beside the questions that relate to
the supporter program.

Rework form with error messages & basic validation

Since PayPal cannot seem to be cajoled into verifying a minimum amount,
we have to do it here with Javascript. This isn't perfect validation:
the form can currently still be submitted with an amount less than $120,
but at least this way Javascript-enabled browsers might prevent some
folks from doing that.