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
When the user changes which addresses mu should consider 'personal',
mu's store should be reinitialized.
After this change, the activation script parses the previously
configured list of addresses and compares it with the new one. If they
differ, it runs the init command even when the store has already been
initialized.
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.
