Turn contents of services.nix into modules

[Chris Warburton]

The whole "monitoredService" and kin work well enough, but they just add
complexity to the system.

We should pull out each of these services into its own NixOS module. I've
already done this with pi-mount (now services.sshMounts).

In the future we might want to investigate systemd's native
monitoring/restarting abilities, rather than using our own polling scripts.
That can wait though. (The reason we don't use the systemd stuff at the moment
is that services can end up disabled if they "fail" a certain number of times in
a row; for our purposes those aren't "failures", they're just state changes;
e.g. if we go offline we shouldn't have SSHFS mounted; when we get back online
they should be mounted again; etc.)