(core) Adding GristConnect login system

Summary:
New login system to allow simple SSO flow that is based on Discourse description that is available at:
https://meta.discourse.org/t/discourseconnect-official-single-sign-on-for-discourse-sso/13045

Test Plan: New core test.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3418

app/gen-server/entity/User.ts

@@ -47,6 +47,9 @@ export class User extends BaseEntity {
   @Column({name: 'options', type: nativeValues.jsonEntityType, nullable: true})
   public options: UserOptions | null;
 
+  @Column({name: 'connect_id', type: String, nullable: true})
+  public connectId: string | null;
+
   /**
    * Get user's email.  Returns undefined if logins has not been joined, or no login
    * is available