[2.0] Ability to make authorization by gate

[slavarazum]

Provide ability to Horizon dashboard access by gate. Like it possible in Nova and Telescope.

Gate::define('viewHorizon', function ($user) {
    return true;
});

[taylorotwell]

I think this would require a bit more. Currently Telescope and Nova export their own service providers into the application structure to make this a bit easier and obvious.