FEATURE: Introduce site setting to allow for non staff pm tagging (#16671)

Currently the only way to allow tagging on pms is to use the `allow_staff_to_tag_pms` site setting.  We are removing that site setting and replacing it with `pm_tags_allowed_for_groups` which will allow for non staff tagging. It will be group based permissions instead of requiring the user to be staff.

If the existing value of `allow_staff_to_tag_pms` is `true` then we include the `staff` groups as a default for `pm_tags_allowed_for_groups`.

lib/imap/sync.rb

@@ -407,7 +407,7 @@ module Imap
     end
 
     def tagging_enabled?
-      SiteSetting.tagging_enabled && SiteSetting.allow_staff_to_tag_pms
+      Guardian.new(Discourse.system_user).can_tag_pms?
     end
   end
 end