Integrate child_type to New Page modal

[rhukster]

The child_type: frontmatter option is an admin-specific option that you can set via the Overrides, and the intention is to all you to set a default child type in the parent page. For example, in blog.md you could set this item and then when creating a new page under blog, it should automatically be set to item.

Currently this is being overridden by the new page modal. The new page modal is forcing a type to be set and is not taking into account the parent type. When the parent field is changed in this new page modal, the Page::childType() should be checked (or this should be checked while building the parent select), and then the value of the template should be changed according if a value is found for that parent.

[w00fz]

This is now implemented in the feature and can be tested/merged.