Merge branch 'refactor-vote' into 'master'

Remove id from vote model

See merge request arsnova/arsnova-lite!222

src/app/models/vote.ts

 export class Vote {
-  id: string;
   userId: string;
   commentId: string;
   vote: number;
@@ -7,7 +6,6 @@ export class Vote {
   constructor(userId: string ,
               commentId: string,
               vote: number) {
-    this.id = '';
     this.userId = userId;
     this.commentId = commentId;
     this.vote = vote;