Make the file tree extendable and compact

[k1sul1]

Each folder has to fold so the menu has room for more items, and #8 needs to be able to inject stuff under the current page.

Probably requires refactoring <Navigation /> from App to Article and cleaning up a bit to make data flow between components (Article -> Navigation).

Maybe build the file tree earlier and use the tree for article fetching also instead of relying on the filename and path from require.context.