(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/plugin/StorageAPI.ts

@@ -0,0 +1,8 @@
+// subset of WebStorage API
+export interface Storage {
+  getItem(key: string): any;
+  hasItem(key: string): boolean;
+  setItem(key: string, value: any): void;
+  removeItem(key: string): void;
+  clear(): void;
+}