adjust permissions on Quassel core files

The SSL certificate/key should have been 600 to ensure its safety,
and the system-level config files should be 644 instead of 640.
Neither of these are a really big deal, but it's good to keep little
things like this in order.
add provisional Tarsnap roles

The first role (tarsnap) just downloads and installs Tarsnap, along
with Tarsnapper (my wrapper of choice).

The second role (tarsnap-nightly) installs some extra systemd unit
files that let you run nightly backups.

Neither are quite tested yet -- everything transfers properly, but
I have no data to actually back up with them.