Enforce all blueprints to be placed under their index page / support multiple index pages per space

Our very early versions of Blueprints enforced structure.

For example, no matter where you were in the space, if you said Add > Meeting, it always went under the meeting notes index page.

We had a bit of feedback about this from customers who typically had the "multiple projects per space" use case that this was frustrating. As an example, they would want a "file list" for every project. So they always had to move the page they created to under the appropriate project page.

We feel that the primary problem here stems from the fact that Blueprints can only have one index page per space. If we had the ability to create multiple index pages per space, we could always enforce that blueprints are created under their index page and enforce this structure.