(core) Persist forks in home db

Summary:
Adds information about forks to the home db. This will be used
later by the UI to list forks of documents.

Test Plan: Browser and server tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3772

app/common/ActiveDocAPI.ts

@@ -169,6 +169,7 @@ export interface QueryResult extends TableFetchResult {
  * docId of XXXXX~FORKID[~USERID] and a urlId of UUUUU~FORKID[~USERID].
  */
 export interface ForkResult {
+  forkId: string;
   docId: string;
   urlId: string;
 }