This commit converts the Backups page in the admin interface
to follow our new admin interface guidelines.
As part of this work, I've also made `AdminPageHeader` and `AdminPageSubheader`
components that can be reused on any admin page for consistency, that handle
the title and action buttons and also breadcrumbs.
Also renamed `AdminPluginFilteredSiteSettings` to `AdminFilteredSiteSettings` since
it can be used generally to show a subset of filtered site settings, not only
settings for a plugin. Not sure if it's ideal to have to define a new route for this
for every config area, but not sure how else to do it right now.
* 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.
