refactor(mu4e): org-msg config

- Simplify advice.
- Fix load-order of org-msg and :config (which use-packages' :after
  convolutes).
- Remove dummy functions (and guard where they were formerly relied on;
  this is a stopgap solution though).

modules/email/mu4e/autoload/email.el

@@ -281,7 +281,8 @@ attach a file, or select a folder to open dired in and select file attachments
 When otherwise called, open a dired buffer and enable `dired-mu4e-attach-ctrl-c-ctrl-c'."
   ;; TODO add ability to attach files (+dirs) as a single (named) archive
   (interactive "p")
-  (+mu4e-compose-org-msg-handle-toggle (/= 1 files-to-attach))
+  (when (fboundp '+mu4e-compose-org-msg-handle-toggle)
+    (+mu4e-compose-org-msg-handle-toggle (/= 1 files-to-attach)))
   (pcase major-mode
     ((or 'mu4e-compose-mode 'org-msg-edit-mode)
      (let ((mail-buffer (current-buffer))