(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/browserInfo.ts

@@ -0,0 +1,13 @@
+import * as Bowser from "bowser"; // TypeScript
+
+let parser: Bowser.Parser.Parser|undefined;
+
+function getParser() {
+  return parser || (parser = Bowser.getParser(window.navigator.userAgent));
+}
+
+// Returns whether the browser we are in is a desktop browser.
+export function isDesktop() {
+  const platformType = getParser().getPlatformType();
+  return (!platformType || platformType === 'desktop');
+}