Choose view template during create page process

I added an extra step for choosing the view-template first and get all the correct page-parts immediately, while creating a new page.
I don't think this is a responsibility of the model, it should live in the controller for now as this is the only place it's ever used.

```ruby
private
def assign_default_page_parts
  Pages.default_parts_for(self).each_with_index do |page_part, index|
    @page.parts << PagePart.new(title: page_part, position: index)
  end
end
```

Agree/disagree?