Pre-fill and hide the "A friendly message" field for PayPal donations

[oadaeh]

On the donation page there is a field labeled "A friendly message?" It is required and not at all clear what it is for. The name should be improved and/or a field description should be added to say what it is for to make it more clear.

[jenlampton]

Paypal has a required field that we aren't using for anything, so it really doesn't matter what goes in there, as long as something does.

In a perfect world we could add some text in there with JavaScript and then hide the field. If anyone has time to work on this, a PR would be welcome :)