(core) Adding new column in users table "ref" with unique identifier.

Summary:
There is a new column in users table called ref (user reference).
It holds user's unique reference number that can be used for features
that require some kind of ownership logic (like comments).

Test Plan: Updated tests

Reviewers: georgegevoian, paulfitz

Reviewed By: georgegevoian, paulfitz

Differential Revision: https://phab.getgrist.com/D3641

app/common/LoginSessionAPI.ts

@@ -8,10 +8,11 @@ export interface UserProfile {
   loginMethod?: 'Google'|'Email + Password'|'External';
 }
 
-// User profile including user id.  All information in it should
+// User profile including user id and user ref.  All information in it should
 // have been validated against database.
 export interface FullUser extends UserProfile {
   id: number;
+  ref?: string|null; // Not filled for anonymous users.
   allowGoogleLogin?: boolean; // when present, specifies whether logging in via Google is possible.
 }