naming: prefer displayName over cn/sn

Now we have on-boarded users, move away from being compatible with the authenticator to using displayName in preference to cn/sn.

In local testing here, this change affects just shy of 20% of users in Google so this should be paired with a manual synchronisation run.

INFO:gsuitesync:Loading configuration    
INFO:gsuitesync.sync:Performing synchronisation in READ ONLY mode.    
INFO:gsuitesync.gapiauth:Loading Google account credentials from "./credentials.json"    
INFO:gsuitesync.sync:Reading eligible user entries from LDAP    
INFO:gsuitesync.sync:Total LDAP entries: 59932    
INFO:gsuitesync.sync:Reading managed user entries from LDAP    
INFO:gsuitesync.sync:Total managed user entries: 59932    
INFO:gsuitesync.sync:Getting information on Google domain users    
INFO:gsuitesync.sync:Ignoring users whose organization unit path matches '^/Service Accounts$'    
INFO:gsuitesync.sync:Total Google users: 61558    
INFO:gsuitesync.sync:Suspended Google users: 1626    
INFO:gsuitesync.sync:Calculating updates...    
INFO:gsuitesync.sync:Number of existing users to update: 12034    
INFO:gsuitesync.sync:Number of users to add: 0    
INFO:gsuitesync.sync:Number of users to reactivate: 0    
INFO:gsuitesync.sync:Number of users to suspend: 0    
INFO:gsuitesync.sync:Configuration will modify 19.55% of users    

Closes #5 (closed)