fix #4393: ui: storage backup view: make pbs-specific columns sortable

fix #4393: ui: storage backup view: make pbs-specific columns sortable

The sort order is analogous to how it behaves in the datastore content
overview in pbs.

This means sorting in ascending order behaves as follows:

Verify State
* failed
* none
* ok

Encryption
* no
* yes

For the encryption state there is theoretically a distinction between
signed and encrypted, but as far as I can tell we do not render this
distinction in PVE, which is why I chose to not make this distinction
for sorting as well.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>