Invite new users via contacts/SMS

[bizzbyster]

There is already this issue #2181 which describes inviting a new user via the "/invite" command. But it would also be great if users could be invited via contacts and including via sending the invite via SMS. This is a standard feature on many other chat applications.

[rasos]

An even more simple way to invite people would be to send a link to my private room, which they simply click on and are asked to login/register. However, RC answers in such a case with a 404 instead of a nice welcome page with a login button, see #8535 . Fixing this is a pre-condition for the "/invite" command.