Allow for private or invite-only registration

[arteymix]

Some registries prefer having a private mechanism for registering users. The idea is that we will generate a secret token used for accessing the /registration endpoint.

Once a user has been registered, the token will be destroyed.

Tokens could be tied to a registration email, preventing one from using a token against a different email address.

Administrators would have the ability to invite users based on a list of email addresses.

Users could also have the ability to invite other users, though on a one-by-one basis.

• add a configuration option to make registration private (or invite-only)
•  hide registration links from the login page and other places
• add an admin section to generate registration tokens & links
• add a mechanism for sending invite emails with a template message
• allow registered users to create invitation links