* 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.
This commit moves the calendar date and time picker shown in
the local dates modal into a core component that can be reused
in other places. Also add system specs to make sure there isn't
any breakages with this feature, and a section to the styleguide.
Martin Brennan