Fix mbox-to-Maildir conversion... again.

Commit ee5671fc fixed mbox-to-Maildir conversion for users upgrading
from Evolution 2.32, who had already migrated to XDG Base Directories.

But turns out, mbox-to-Maildir conversion was still broken for users
coming from Evolution 2.30 or earlier because the logic to move files
into XDG Base Directories was running *after* the conversion routine.
So the conversion routine found nothing to convert, and users were
left with a broken "On This Computer" mail store.

This commit runs the XDG Base Directory migration first on startup,
followed by the mbox-to-Maildir conversion.

(cherry picked from commit 1723ee09)