Add optional signup

Adds an option to gitlab config that will enable user signup. By default, signup is disabled.
When signup is enabled, email that user receives on signup doesn't contain the password for security reasons.

This option is useful for companies and universities where administrator cannot add all users manually.
https://groups.google.com/d/msg/gitlabhq/ANAmyqP4m5U/ZqBGEWWVbhsJ