StorageService paths should be treated as opaque strings, part 1 (#5532)

First step to fix #5068 by treating StorageService paths as opaque
strings, by telling the client the canonical path when performing a
directory listing. This enables clients to solve for the file's own
name.

In order to maintain compatibility with the current web-client-ui, all
paths must still begin with "/" - the final patch for this issue will
remove that limitation.

Also includes the file separator, notebook directory, and layouts
directory as configuration details for clients to read.

Partial #5068