(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/common/BrowserSettings.ts

@@ -0,0 +1,7 @@
+/**
+ * Describes the settings that a browser sends to the server.
+ */
+export interface BrowserSettings {
+  // The browser's timezone, must be one of `momet.tz.names()`.
+  timezone?: string;
+}