Limiting page/template visibility

On bigger sites the admin is very hard to use due to two things:
1. There is no way to hide pages that possibly cant have children (blogpost). To put it other way around - there is no way to say in blueprint that this page cant have children. It gets extremely messy very quickly - when you are making child page of some root page ordered after your blog folder for example you scroll hundreds blogposts to finally find it.
2. Similar thing with templates - there is no way of limiting what kind of template children of page can have. If you have 30 templates on site you have to know and find the desired template for blogpost every single time. This could be filtered when you select your root page. When i select root folder BLOG i will 100% want to select blogpost template.

This puts very high demands on the users/editors because they realy have to learn it. From my experience they mess up all the fucking time.

The issue names no files or tests. Start by tracing the admin page-creation flow and blueprint/template selection logic, then determine where child-page and template restrictions are represented. Done means editors can limit child visibility and allowed child templates per parent or blueprint, with coverage for both behaviors.

Written by the indexing model from the issue text.