Add generic currency fallback with optional notification

Add FallbackCurrency and NotifyOnConvert gateway globals to enable
converting unsupported currencies to a default and optionally notfying
the donor.

Moved the conversion logic from PaypalGateway to new GatewayForm
method fallbackToDefaultCurrency.  GatewayForm::validateForm calls
this and notifies if conversion has occurred and either notification
is enabled or any other form error exist.

Change-Id: I370391e7f20f5f3783acfc00f3ec94430ba87e77