Loading sessions with file navigation and completion

Recently, there was a change to save sessions under their template name instead of simply at the top level at `mindstream-save-session-path`.

This ended up breaking `mindstream-load-session` since this was still listing top level contents of `mindstream-save-session-path` by name. This was fixed to show the contents of every top level folder by listing the contents of each folder and then appending them all together, but the drawbacks of this approach are:

- the user cannot navigate to paths other than `mindstream-save-session-path`
- the sessions are listed as absolute paths (rather than e.g. session names) which are harder to read
- if a session is stored in a nested path or at the top level, it isn't possible to navigate to it since we have hardcoded the listing of toplevel+1 directory contents

It could be more natural for `mindstream-load-session` to show a file navigation selection menu that starts at `mindstream-save-session-path` and allows the user to navigate to the session they want to load (which may even be outside the save session path). But if we do it this way, then it seems as though we would lose the ability to have recent completion candidates displayed, so that's a tradeoff we'd be making.

To have the best of both, we could potentially modify `mindstream-load-session` to be file navigation-based, and introduce a separate feature called `mindstream-load-recent-session` which simply lists all recent sessions by name, and maps them under the hood to their absolute paths before loading them.