Prefix site configs

Renamed all of the "ukin" and other fco site and mappings files to be prefixed with "fco_".

Also asserts the same naming convention for the businesslink and directgov site files, mappings and hand-crafted nginx configurations.

This clarifies the owner of each transitioned domain, and simplifies counting the number of mappings per-department.

We could separate these into a sub-directory for each site, but that makes things more complicated for the majority of sites which have only one file. You need a symbolic name for the nginx sites-enabled directory and in other places anyway.