Issue 1331: Account creation options

Resolves issue: http://code.google.com/p/otwarchive/issues/detail?id=1331

Created the 'creation_requires_invite' setting on the admin_settings table. If account creation is enabled AND 'creation_requires_invite' is enabled then the user is given a message saying that they will need an invite, and are deposited on the Invite queue page. 
"Creation" probably doesn't need to be capitalised here (the corresponding test will need amending if you change this)