(core) move client code to core

Summary:
This moves all client code to core, and makes minimal fix-ups to
get grist and grist-core to compile correctly.  The client works
in core, but I'm leaving clean-up around the build and bundles to
follow-up.

Test Plan: existing tests pass; server-dev bundle looks sane

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2627

app/client/lib/download.js

@@ -0,0 +1,31 @@
+const G = require('../lib/browserGlobals').get('document');
+const dom = require('../lib/dom');
+
+/**
+ * Note about testing
+ * It is difficult to test file downloads as Selenuim and javascript do not provide
+ * an easy way to control native dialogs.
+ * One approach would be to configure the test browser to automatically start the download and
+ * save the file in a specific place. Then check that the file exists at that location.
+ * Firefox documentation: http://kb.mozillazine.org/File_types_and_download_actions
+ * Approach detailed here in java: https://www.seleniumeasy.com/selenium-tutorials/verify-file-after-downloading-using-webdriver-java
+ */
+
+let _download = null;
+/**
+ * Trigger a download on the file at the given url.
+ * @param {String} href: The url of the download.
+ */
+function download(href) {
+  if (!_download) {
+    _download = dom('a', {
+      style: 'position: absolute; top: 0; display: none',
+      download: ''
+    });
+    G.document.body.appendChild(_download);
+  }
+  _download.setAttribute('href', href);
+  _download.click();
+}
+
+module.exports = download;