Use $app to get the current user

[mlocati]

Currently

$u = new User();

and

$u = $app->make(User::class);

are almost the same.

BTW, because https://github.com/concrete5/concrete5/pull/7433 has been merged into the core, using the $app->make(User::class) approach will be faster than using new User() starting from concrete5 version 8.6, so it's worth using that approach.