DEV: Add more structure for admin plugin config nav (#26707)

* Simplify config nav link generation to always inject the Settings
  tab
* Auto-redirect to the first non-settings config link (if there is one)
  when the user lands on /admin/plugins/:plugin_id
* Add `extras` to admin plugin serializer so plugins can add more
  data on first load
* Add PikadayCalendar page object for system specs, extracted from the
CalendarDateTimePicker to make it more generic.

app/serializers/admin_plugin_serializer.rb

@@ -68,7 +68,7 @@ class AdminPluginSerializer < ApplicationSerializer
 
     ret = route.slice(:location, :label)
     if route[:use_new_show_route]
-      ret[:full_location] = "adminPlugins.show.#{ret[:location]}"
+      ret[:full_location] = "adminPlugins.show"
       ret[:use_new_show_route] = true
     else
       ret[:full_location] = "adminPlugins.#{ret[:location]}"