* 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 changes the API for registering the plugin config
page nav configuration from a server-side to a JS one;
there is no need for it to be server-side.
It also makes some changes to allow for 2 different ways of displaying
navigation for plugin pages, depending on complexity:
* TOP - This is the best mode for simple plugins without a lot of different
custom configuration pages, and it reuses the grey horizontal nav bar
already used for admins.
* SIDEBAR - This is better for more complex plugins; likely this won't
be used in the near future, but it's readily available if needed
There is a new AdminPluginConfigNavManager service too to manage which
plugin the admin is actively viewing, otherwise we would have trouble
hiding the main plugin nav for admins when viewing a single plugin.
This commit adds new plugin show routes (`/admin/plugins/:plugin_id`) as we move
towards every plugin having a consistent UI/landing page.
As part of this, we are introducing a consistent way for plugins
to show an inner sidebar in their config page, via a new plugin
API `register_admin_config_nav_routes`
This accepts an array of links with a label/text, and an
ember route. Once this commit is merged we can start the process
of conforming other plugins to follow this pattern, as well
as supporting a single-page version of this for simpler plugins
that don't require an inner sidebar.
Part of /t/122841 internally
