Subscriptions for monthly supporters.

There are now two options at the bottom of the page, annual and monthly
supporters.

In addition, there is Javascript code to cause the annual and monthly
items disappear and reappear upon selection either in the donate box or
the selector above the items.

I tested this in links and it seems to degrade reasonably well.

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.