implement logic for deregistration period

The deregistration period should be communicated to users. This means:

Case 1: Registration Start (R) --> Deregistration End (D) --> Registration End (E)

• Before R: status quo
• Between R and D: "You are signed up and got a spot. Deregistration is possible until [Date]"
• Between D and E: "You are signed up and got a spot." + disabled Button mit "The deregistration period is over"

Case 2: Registration Start (R) --> Registration End (E) --> Deregistration End (D)

• Before R: status quo
• Between R and E: "You are signed up and got a spot. Deregistration is possible until [Date]"
• Between E and D: "You are signed up and got a spot. Deregistration is possible until [Date]" + Button "Signoff" + disabled Button "The registration period is over"

For this, I also had to move the deleteAction a bit up for the "Between E and D" case.