Add email confirmation option for new registrations

[akrherz]

...Migrated from Ignite Jira OF-1276...

Ignite Forum Post

It can use current Registration plugin's option or it could probably use some REST API query to do that. For email sending it can use Email Settings part already in Openfire. In the case of Registration plugin it would use the internal Jetty web service and admin will have to expose Admin Console to the Internet. There is probably a way to integrate Openfire with some existing web service (Apache, etc.), but that sounds more complicated.