[5.4] Add ability to remove a global scope with another global scope

[KKSzymanowski]

As described in #19282, there is currently no way to remove a global scope with another global scope.
Example use case would be to automatically allow users with elevated privileges to view soft-deleted models.

class Product extends Model 
{

    use SoftDeletes;

    protected static function boot()
    {
        static::addGlobalScope(function(Builder $query) {
            if (auth()->user()->hasElevatedPrivileges()) {
                $query->withoutGlobalScope(SoftDeletingScope::class);
            }
        });

        parent::boot();
    }
}

This allows for replacing:

class ProductController extends Controller
{
    public function show($productId, Request $request)
    {
        if ($request->user()->hasElevatedPrivileges()) {
            return Product::withTrashed()->findOrFail($productId);
        }

        return Product::findOrFail($productId);
    }
}

with

class ProductController extends Controller
{
    public function show(Product $product)
    {
        return $product;
    }
}

[taylorotwell]

Feels like a pretty hacky thing to do in your app but ok, heh.

[devcircus]

Definitely hacky. Doesn't feel laravelish.

[KKSzymanowski]

@taylorotwell, @devcircus What approach would you recommend?

[devcircus]

@taylorotwell merged it so I guess that would be the approach I'd take now. The part that seems hacky to me is the need for something like this in your app. Laravel devs often need to conditionally apply different scopes, yet I've never heard of this approach. Actually, to me, global scoping seems like the wrong approach if you're needing to conditionally apply it or alternate with another scope.
However, now that it's baked in to the framework, run with it.

[KKSzymanowski]

Well, it felt a lot easier to strip down every conditional call to withTrashed from controllers and replace it with one global scope removal.

I don't think this PR was merged because it is the correct approach to solving this problem but rather because it was a bug which allowed for applying a scope which shouldn't be applied(didn't exist in the scopes array).

I'm open for a discussion about this. To be honest, removing the global scope after it was added felt weird but at the same time it felt a lot simpler than eg. overwriting SoftDeletes and SoftDeletingScope and adding the where clause under some condition.