(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/migration/1652277549983-UserConnectId.ts

@@ -0,0 +1,22 @@
+import {MigrationInterface, QueryRunner, TableColumn, TableIndex} from "typeorm";
+
+export class UserConnectId1652277549983 implements MigrationInterface {
+  public async up(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.addColumn("users", new TableColumn({
+      name: "connect_id",
+      type: 'varchar',
+      isNullable: true,
+      isUnique: true,
+    }));
+    await queryRunner.createIndex("users", new TableIndex({
+      name: "users_connect_id",
+      columnNames: ["connect_id"],
+      isUnique: true
+    }));
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.dropIndex("users", "users_connect_id");
+    await queryRunner.dropColumn("users", "connect_id");
+  }
+}