This commit attempts to improve the mobile experience for
admin page header and subheader by automatically collapsing
all action buttons in these components into a DMenu when viewing
mobile.
This is done by using different "list" wrapper components and a
DMenu trigger and a DropdownMenu on mobile only, and uses has-block
to determine whether to render the DMenu trigger at all.
This also removes the `PluginOutlet` in `AdminPluginConfigPage`, it
was too inflexible for this `DropdownMenu` case, and since the `:actions`
were always rendering we couldn't rely on `has-block`. A new plugin API,
`registerPluginHeaderActionComponent`, has been introduced instead to
replace it.
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.
