Move to node fetch API for web requests instead of XHR (#1552)

* Use node-fetch instead of xhr

* Remove XMLHttpRequest.js

// FREEBIE

* Avoid calling json() on non json responses

Previously we would catch and swallow JSON parsing errors resulting from an
empty response, though empty responses are normal from a few endpoints, like
requesting sms or voice registration codes.

Since the JSON parsing call is now handled internally by node-fetch, we have to
keep closer track of our expected response type to avoid throwing an exception.

// FREEBIE

Gruntfile.js

@@ -101,7 +101,6 @@ module.exports = function(grunt) {
         '!js/Mp3LameEncoder.min.js',
         '!js/libsignal-protocol-worker.js',
         '!js/components.js',
-        '!js/XMLHttpRequest.js',
         '!js/signal_protocol_store.js',
         '_locales/**/*'
       ],
@@ -170,7 +169,6 @@ module.exports = function(grunt) {
         '!js/Mp3LameEncoder.min.js',
         '!js/libsignal-protocol-worker.js',
         '!js/components.js',
-        '!js/XMLHttpRequest.js',
         'test/**/*.js',
         '!test/blanket_mocha.js',
         '!test/test.js',