smoothen registration process and first experience

First approach for https://github.com/openSUSE/osem/issues/215 and https://github.com/openSUSE/osem/issues/209
Because if there is more than one conferences, we cannot really know which conference the user wants to register to. 

I am not sure if that's what you have in mind:
Even if we check which conferences have registrations open, that could still create problems.
Scenario: 
Conference registration is closed. Conference is happening now.  Despite the fact that registration is closed, people might still create an account manually, so that they can do onsite registration from the reg desk.

If at the same there is another conference with open registration, people would register automatically for that conference.

If this is not what you had in mind, I am afraid I am not following. Please elaborate?