Replace sha256.js and base64_encode() with Mozilla's internal equivalents

To reduce code bloat and avoid JavaScript's bit shifting inefficiencies it would be good to replace sha256.js and base64_encode() in ssl-observatory.js with functionality provided by Mozilla itself. The former is quite easy using nsICryptoHash (see the the example "Computing the Hash of a String" on https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsICryptoHash). The latter is not working directly using btoa(). The reason is that |derData| is an array but btoa() needs a string as argument. But one can construct a string out of the array and use that in turn as input. The relevant code snipped would be something like

  let result = "";
  for (let j = 0, dataLength = derData.length; j < dataLength; ++j) {
    result += String.fromCharCode(derData[j]);
  }
  base64Certs.push(btoa(result));

I have implemented both approaches in JonDoFox and they work as expected.