Add customActions property.

[yajra]

This PR will:

1. Add customActions property to simplify editor's extended functionality.
2. Fix HTTP code (422) when an error occurs.

Usage

class DocumentsDataTableEditor extends DataTablesEditor
{
    protected $model = Document::class;

    protected $customActions = ['import'];

    public function import(Request $request)
    {
        // process import
    }