Register user using invite code

"Invite codes":https://leap.se/en/docs/platform/services/webapp#invite-codes has been merged into leap_web, now the client needs to adapt this feature as well. I'll soon activate invite codes on unstable.bm, so client devs can use it for development.

(from redmine: created on 2015-10-23, relates #6982 (closed), relates #8505 (closed))