Add quick links to the admin interface in the WebUI (#8545)

* Allow to show a specific status in the admin interface

* Let the front-end know the current account is a moderator

* Add admin links to status and account menus

If the current logged-in user is an admin, add quick links to the admin
interface in account and toot dropdown menu. Suggestion by @ashkitten

* Use @statuses.first instead of @statuses[0]

app/serializers/initial_state_serializer.rb

@@ -29,6 +29,7 @@ class InitialStateSerializer < ActiveModel::Serializer
       store[:display_media]   = object.current_account.user.setting_display_media
       store[:expand_spoilers] = object.current_account.user.setting_expand_spoilers
       store[:reduce_motion]   = object.current_account.user.setting_reduce_motion
+      store[:is_staff]        = object.current_account.user.staff?
     end
 
     store