Update catalog view to display category listing as a tree

This proposal is **not** final and will be revisited:

* the `category` metadata may capture the structure by separating parent category from child category using a dot `.`.
* for now an extension is associated with a single category (see [here](https://github.com/girder/slicer_package_manager/blob/addd15cd74d6a5bbbe5fb7a5338d76136926817c/server/api/app.py#L557)), we may visit this

Start with server/api/app.py around line 557, which the issue cites for the current single-category association. Before implementation, clarify whether category metadata should encode parent and child categories with a dot and define the intended tree behavior; the proposal explicitly says it is not final.

Written by the indexing model from the issue text.