Reconsider use of `os` and `glob` modules for local personas search due to security and compatibility

[andrii-i]

1. There is an added security risk with loading any module matching the glob .jupyter/personas/*persona*.py. We should explore whether we can be more secure by default.

2. We are using the os and glob module to find persona files on the server's local filesystem. This will not work for Jupyter users with a custom/remote ContentsManager configured in their Jupyter Server.

Originally posted by @dlqqq in #1443 (review)