Allows for the database path for mu to be configured. Useful for keeping
the maildir and mu xapian cache together without having to modify
XDG_CACHE_HOME. Add test to check for custom home setting.
Fixes#5534
This has no effect if the user does not have any aliases defined for
any accounts.
This will also only add `--my-address=` to only accounts that are
enabled to be tracked by mu.