Filesystem KVS with Roster source `all` is broken

[bradley-erickson]

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()