ProfileType: moved adding 'current_password' to protected field to allow override

The `ProfileType` form type adds a 'current_password' field by default, but that might not always be desired. This PR makes it easier to override the field by extracting the code into a protected method.

So, instead of overriding the whole `buildForm()` method, a subclass can include just this:

```
    protected function addCurrentPasswordToForm(FormBuilderInterface $builder)
    {
    }
```