(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/server/lib/ILoginSession.ts

@@ -4,6 +4,7 @@ import {Client} from 'app/server/lib/Client';
 export interface ILoginSession {
   clients: Set<Client>;
   getEmail(): Promise<string>;
+  getSessionProfile(): Promise<UserProfile|null>;
   // Log out
   clearSession(): Promise<void>;