Create user last connection datetime (#935)

Each time the a Grist page is reload the `last_connection_at` of the user is updated

resolve [#924](https://github.com/gristlabs/grist-core/issues/924)

app/gen-server/entity/User.ts

@@ -29,6 +29,9 @@ export class User extends BaseEntity {
   @Column({name: 'first_login_at', type: Date, nullable: true})
   public firstLoginAt: Date | null;
 
+  @Column({name: 'last_connection_at', type: Date, nullable: true})
+  public lastConnectionAt: Date | null;
+
   @OneToOne(type => Organization, organization => organization.owner)
   public personalOrg: Organization;