It gets spuriously closed sometimes.

All card actions are defined via the page and container abstractions,
which will put them into the kebab menu of table rows.  But then,
dangerous and impossible actions are removed from that menu.

We could do the same in other places.

To be able to link to a BTRFS device in the BTRFS volume page we want to
register all backing blocks to create link to btrfs volume devices.

This introduces the concept of BTRFS volumes in the storage page, a
BTRFS volume can have multiple device(s). The first version won't
implement adding/removing devices but should support showing them and
making sense of it. We also show the btrfs subvolumes of a volume and
should in the future allow creating of subvolumes under a subvolume and
the root.