Merge branch '44-comment-creation-participant-model' into 'master'

Resolve "comment creation - participant model"

Closes #44

See merge request swtp-block-ws17/arsnova-angular-frontend!26

src/app/comment.ts

+export class Comment {
+  id: string;
+  roomId: string;
+  revision: string;
+  subject: string;
+  body: string;
+  read: boolean;
+  creationTimestamp: Date;
+}