(core) Add account page option to allow Google login

Summary:
Enabled by default, the new checkbox is only visible to
users logged in with email/password, and controls whether it is possible
to log in to the same account via a Google account
(with matching email). When disabled, CognitoClient will refuse logins
from Google if a Grist account with the same email exists.

Test Plan:
Server and browser tests for setting flag. Manual tests to verify
Cognito doesn't allow signing in with Google when flag is disabled.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3257

app/gen-server/entity/User.ts

@@ -1,3 +1,5 @@
+import {UserOptions} from 'app/common/UserAPI';
+import {nativeValues} from 'app/gen-server/lib/values';
 import {BaseEntity, Column, Entity, JoinTable, ManyToMany, OneToMany, OneToOne,
         PrimaryGeneratedColumn} from "typeorm";
 
@@ -42,6 +44,9 @@ export class User extends BaseEntity {
   @Column({name: 'is_first_time_user', default: false})
   public isFirstTimeUser: boolean;
 
+  @Column({name: 'options', type: nativeValues.jsonEntityType, nullable: true})
+  public options: UserOptions | null;
+
   /**
    * Get user's email.  Returns undefined if logins has not been joined, or no login
    * is available