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.
What do you think about that?

pages-controller:

```ruby
def new
  @page = Page.new(new_page_params)
  @page.view_template = @valid_view_templates.first if @valid_view_templates.length == 1
  render "select_view_template" and return if @page.view_template.blank?
  @page.assign_default_parts
end
```
page-model:

```ruby
def assign_default_parts
  Pages.default_parts_for(self).each_with_index do |page_part, index|
    self.parts << PagePart.new(title: page_part, position: index)
  end
end
```