lang/org: use org-attach instead of custom system

- Adds more keys under `<localleader> a`
- Reconfigures org-download to use org-attach system
- Make org-id-track-globally = t a global default now (not just for
  org-brain)
- Renamed +org-init-centralized-attachments-h to +org-init-attachments-h
- org-attach-store-link-p = t (stores a link to an attachment when it is
  attached)
- org-attach-use-inheritance = t (inherit attachment properties from
  parent nodes)

modules/lang/org/contrib/dragndrop.el

@@ -10,7 +10,8 @@
             '("^data:" . org-download-dnd-base64))
   (advice-add #'org-download-enable :override #'ignore)
   :config
-  (setq org-download-image-dir org-attach-directory
+  (setq org-download-image-dir org-attach-id-dir
+        org-download-method 'attach
         org-download-heading-lvl nil
         org-download-timestamp "_%Y%m%d_%H%M%S"
         org-download-screenshot-method