Add a "lockdown mode" where all new IFDB accounts require moderator approval

[dfabulich]

@alice-blue suggested an idea:

I wondered if it would have made sense to set up a “manual approval only” mode. So if spam starts getting really bad on a given day, mods could pull a lever that requires all new accounts, regardless of domain, to have manual approval before activation. And then mods could move the lever back when the spammers slow down.