UX: multiple drafts menu improvements (#31195)

This change includes the following updates:

- Rename view all to view all drafts
- Remove view all link from drop-down when all drafts are displayed in
the menu
- Different icon for draft topics and PMs (adds envelope for PMs)
- Disable drop-down when New Topic button is disabled (private
categories etc)
- Improve drafts drop-down loading (no longer disables the trigger btn
on click)

config/locales/client.en.yml

@@ -500,7 +500,7 @@ en:
       dropdown:
         title: "Open the latest drafts menu"
         untitled: "Untitled draft"
-        view_all: "view all"
+        view_all: "view all drafts"
         other_drafts:
           one: "+%{count} other draft"
           other: "+%{count} other drafts"