snomasks for all account (un)registration events

[slingamn]

From review feedback on #345; we should send a snomask for account registration and unregistration.

Hmm, we should have a snomask here note that an oper's registered an account through SAREGISTER (specifying the oper's name/hostmask and the name of the account they've created). It's something we could look at later though if we could just create an issue for that (tbh I need to add one as well for account registration more generally if it doesn't already spit one out), e.g. something along the lines of

 client.server.snomasks.Send(sno.LocalAccounts, fmt.Sprintf(ircfmt.Unescape("Client $c[grey][$r%s$c[grey]] logged into account $c[grey][$r%s$c[grey]]"), client.nickMaskString, account))