Don't partition on user's behalf

[rushmorem]

Just like NixOS, we should assume that partitions have already been created and configured correctly. We should then simply mount the partition if it's not already mounted and install NixOS on it. The partition that we install NixOS into must either be blank or must already contain NixOS. This will help quite a bit in protecting ourselves from trashing important data.

[rushmorem]

Since this is partly fixed I'm reopening this issue. We no longer do partitioning but we don't verify yet whether the partition is a NixOS one or it's blank.