luci-mod-admin-full: check backup.tar.gz on apply

luci-mod-admin-full: check backup.tar.gz on apply

If an uploaded backup.tar.gz is not valid we will not get a respond from
LuCI. The system will perform a reboot without applying the "tar.gz"
even though the backup import failed.

To fix this check if the backup archive is valid with the command
"gunzip -t <archive>" and if the validation fails render the flashops page
with a hint. On the other hand apply the backup archive and perform a
reboot as before.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>