add option to prevent unapproved members to log in

[cherbst]

This adds an option to the settings page to completely lock the site for users who are still in moderation.

[tw2113]

Not completely set on this idea, but will let it stew in my head, as well as do some testing before committing for sure.

See issue https://github.com/WebDevStudios/BuddyPress-Registration-Options/issues/156 for tracking