Limit new users registration

As said in https://github.com/sharelatex/sharelatex/issues/119 there is the need to limit the registration of new users when runnning sharelatex in your own organization server. The approach implemented here is very simple. A "organization password" can be set in the configfile. This password is required during the registration procedure. So in my case I will set a password and say it to all the members of my lab, so only they can create users in our locally hosted sharelatex.

I think it would be nice that in the case the password is not set in the config file, the form doesn't even show the field, as it might confuse the new users.Sorry, but I didn't manage to access the setting variables from the jade files.