Deny votes for own group

[leaantonia]

User Story:
As a host I don't want my participants to be able to vote for themselves in a group vote to get fairer results.
Acceptance Criteria:
General:
do after #231

[vogelingmar]

Acceptance Criteria: The option that is your own group is greyed out and therefore can't be ticked. According to the token the participant cannot vote for their group, because the token is linked to a group.

[florian-str]

Additionally, a user should only be able to see options he/ she can vote for.