minor bug: test for adding donation to template is a boolean on a string instead of a number

In source.js

There is a test:

    // If the user wants to donate some satoshis..
    if (satoshis) {

But satoshis is a string:

    const satoshis = document
      .getElementById("donationAmount")
      .getAttribute("data-satoshis");

So even if it is zero, it still triggers creation of a pledge template.

I can only get a zero into that value through some manual manipulation of document but it seems it can happen in some rare cases as in #104 (closed) .