Filesystem KVS with Roster source `all` is broken

When using the filesystem KVS, the `,` in the key strings are converted to `-` to remain as a valid filename. The `all` roster source fetches all available keys and extracts student ids by splitting on `,`.

Even if students exist in the filesystem, their IDs are not being properly extracted and thus, we end up with an empty class.

We could:

1. Warn the user that these are incompatible setting
2. Create an edge case `learning_observer/rosters.py:all_students` that checks the KVS type and extracts students based on that
3. Modify the FileSystem KVS to use the appropriate key string when returning `.keys()`

Start at learning_observer/rosters.py:all_students and inspect the filesystem KVS implementation and its .keys() behavior; compare the stored filename keys with the comma-splitting logic. Done means the `all` roster source extracts existing student IDs from filesystem-backed KVS data and no longer produces an empty class.

Written by the indexing model from the issue text.