* 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.
