Make use of user_mail_dirs abstraction

Note that this disallows access to ~/mail now.
I know dovecot uses that dir by default. Would mutt need access for any reason?

apparmor.d/profiles-m-r/mutt

@@ -50,8 +50,8 @@ profile mutt @{exec_path} {
   owner @{HOME}/postponed    rwk,
   owner @{HOME}/sent         rwk,
   # User maildir
-  owner @{HOME}/{M,m}ail/    rw,
-  owner @{HOME}/{M,m}ail/**  rwk, 
+  owner @{user_mail_dirs}/ rw,
+  owner @{user_mail_dirs}/** rwlk -> @{user_mail_dirs}/**,
 
   # Trusted certificate store
   owner @{HOME}/.mutt_certificates rwk,