Add ability to delete and create files and dirs

[doesntgolf]

Eventually, the new files we'll want to be able to create through the admin UI are new content files (posts, pages, docs, so on), new layout files, new static files (and dirs?), and new data files.

To create a new section, we'll likely want to have creating a new archetype automatically kickoff the process of creating the corresponding Content and Layout dirs.

For the time being though, it probably make sense to just allow creating and deleting files and dirs everywhere.