owner checks: handle backups owned by API tokens

owner checks: handle backups owned by API tokens

a user should be allowed to read/list/overwrite backups owned by their
own tokens, but a token should not be able to read/list/overwrite
backups owned by their owning user.

when changing ownership of a backup group, a user should be able to
transfer ownership to/from their own tokens if the backup is owned by
them (or one of their tokens).

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>