(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/TestState.ts

@@ -0,0 +1,4 @@
+export interface TestState {
+  clipboard?: string;
+  anchorApplied?: boolean;
+}