Comment 7 for bug 1353516

I've now added a third commit to prevent searching on usernames if the 'nousernames' option is enforced (except for site admins and site staff who can always see usernames anyway, and should therefore also be able to always search on them).

I've also amended the first commit to fix a couple of areas I spotted where the 'searchusernames' option was being used to decide whether or not to *display* usernames. The 'nousernames' option seems a more logical choice so is now used instead.