(core) reconcile webhook and widget description migrations

Summary:
Due to a mishap, two distinct migrations with the same migration
number were introduced into Grist. This diff reconciles them as
best we can, by adding another migration to make sure both desired
changes have run (and running them if not).

Test Plan:
updated a test; checked manually that documents
with different 38 migrations are handled as expected.

Reviewers: georgegevoian, jarek

Reviewed By: georgegevoian, jarek

Differential Revision: https://phab.getgrist.com/D3895

app/common/schema.ts

@@ -4,7 +4,7 @@ import { GristObjCode } from "app/plugin/GristData";
 
 // tslint:disable:object-literal-key-quotes
 
-export const SCHEMA_VERSION = 38;
+export const SCHEMA_VERSION = 39;
 
 export const schema = {
 
@@ -104,6 +104,7 @@ export const schema = {
     parentId            : "Ref:_grist_Views",
     parentKey           : "Text",
     title               : "Text",
+    description         : "Text",
     defaultWidth        : "Int",
     borderWidth         : "Int",
     theme               : "Text",
@@ -314,6 +315,7 @@ export interface SchemaTypes {
     parentId: number;
     parentKey: string;
     title: string;
+    description: string;
     defaultWidth: number;
     borderWidth: number;
     theme: string;