prosody modules: add mod_block_registrations from  https://code.google.com/p/prosody-modules/source/browse/mod_block_registrations/mod_block_registrations.lua and config
 - only allow alphanumeric chars for registrations
 - prevent registrations with some usernames (admin, root, *master...)

TODO: in the menu, allow switching registrations on/off
TODO: in the menu, allow specifying allowed usernames