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.
* `@ember/owner` instead of `@ember/application`
* `discourse-i18n` instead of `I18n`
* `{ service } from "@ember/service"` instead of `inject as service`
This commit makes it so the site settings filter controls and
the list of settings input editors themselves can be used elsewhere
in the admin UI outside of /admin/site_settings
This allows us to provide more targeted groups of settings in different
UI areas where it makes sense to provide them, such as on plugin pages.
You could open a single page for a plugin where you can see information
about that plugin, change settings, and configure it with custom UIs
in the one place.
In future we will do this in "config areas" for other parts of the
admin UI.
