Basic feature implementation

[dbrumann]

Implements the feature by adding each step directly in SecurityController::register.

Refer to each commit to see which change was introduced for which step:

Commit	Description
6da1261	Check if the invite code to be used exists and is still open
46b17c0	Create a new user (basic registration)
596bb2b	Mark the invitation for the code as being redeemed
32eee6d	Create new invite codes for the registered user
30419ed	Notify invite owner, that their code was redeemed