Support nested collections

Currently, it is not possible to create collections inside other collections. I think this could be a useful feature for organizations that need a clearer hierarchy between different spaces.

In our case, we host an Outline instance for students at our school. The goal is to give student clubs a space where they can write and maintain their own documentation. We have almost thirty clubs, and each club has a private collection for its members.

However, each club cannot divide its space into multiple sub-collections. As a workaround, students create documents that act like collections.

I feel this is not the ideal way Outline should be used. I think supporting nested collections would make it easier to structure documentation for teams, or any organization with multiple groups and subgroups.